The Existence of God may be shown by five proofs.  The first is drawn from the principle of motion.  It is evident to our senses that motion exists.  Whatever is moved must be moved by some external agent.  Nothing is moved unless it is in potentiality (in potentia) to its term of motion.  Motion is made accordingly as things are changed from the potential to the actual, and this requires some actual agent to move them from the potential state.  Since it cannot be that anything should be both potential and actual as regards the same order, it follows that the mover and the moved cannot be identical.  Thus, not to go on indefinitely, we must come at last to a First Cause immovable of motion; and there we find God.

The Second Proof consists in the order of Efficient Causes in sensible objects.  Nothing can be its own efficient cause, for then it would exist before itself.  In every order of being the first is the cause of the intermediate, and this latter the cause of the ultimate; so that if the cause e removed the effect ceases to be, and if the first is gone there can e neither the intermediate nor the ultimate.  Hence, not to proceed indefinitely, there must be a First Efficient Cause; and there too we find God.

The Third Proof is taken from possible and necessary things.  Some things may be or not be; they are possible, as they are subject to generation and decomposition; but everything could not be always thus, for what is not necessary at some time is not.  If, therefore, all things may possily not be, at some time there must have been nothing; and if this be true even now, there would be nothing, for what is not can only exist by that which is.  All things, therefore, are not mere possibilities in their origin; there must exist some necessary thing.  But whatever is necessary, either has cause for its necessity or it has not; and, not to proceed indefinitely, as regards necessary things with a cause for their being necessary, we are obliged to postulate something necessary in itself with no cause for its necessity, but itself the cause to other things of their necessity; and this is God.

The Fourth Proof proceeds from our finding some things better than others.  A thing is said to be more or less as it approaches to that which is called the most.  There exists, therefore, something which is best and truest, the source to things of all goodness and truth, and of all their other perfections; and this we call God.

The Fifth Proof is drawn from the idea of government.  Some things are without understanding, yet they work for an end, because often and always they work in the same way to obtain the best end; hence it is evident that they attain the end not by chance, but by intention; and since they must act towards the end not by their own but by some one’s knowledge, they reach the end because they are directed by an Intelligent Being.  There must, therefore, be such an Intelligent Being Who directs all natural things to their end; and Him we call God.

This doctrine is a science proceeding from principles made known to us by the light of a higher science, as music proceeds from principles explained by arithmetic.  For Sacred Doctrine proceeds from principles made known by the light of a higher knowledge, namely, the Divine Knowledge, and in it certain particulars are treated of, both as an example of life and in order that we may know clearly by what instrumentality this revelation is made.

Sacred Doctrine takes account of all things only in so far as they belong to the formal order of Divine revelation.  This science is one, neither wholly practical nor wholly speculative; but being of a higher order it includes both, yet remains one, as God knows both Himself and what He does with the same knowledge.  It is, however, more speculative than piratical, for it treats more of Divine things than of human actions, being concerned with the latter only in so far as they are intended to lead man to the perfect knowledge of God, in which eternal beatitude consists.

And this science is higher in dignity than other speculative sciences, for these derive their certainty from human reason, which may err, while Sacred Doctrine, owing to the light of Divine Knowledge, can never be deceived; moreover, they consider only things which are below reason, while she treats principally of such as transcend reason.  Sacred Doctrine is also higher in dignity than other practical sciences, for, among such, that science is accounted the more honorable which is not subordinated to a further end, as military science is to civil; but the end of this doctrine, in as far as it is practical, is eternal beatitude, to which all other ends of the practical sciences are subordinate.

And since it treats most properly of God as the First Cause, it is wisdom in the highest sense; for he is accounted wise in any department of knowledge who studies the highest cause in that department.  Seeing, therefore, that Sacred Doctrine determines concerning God, not only as he may be known by creatures, but also as He is known by Himself alone, and by others through revelation, it is properly called wisdom in the highest sense, and God is the subject of it, for all things are considered with reference to Him, all things being related to Him either as their beginning or their end.

Sacred Doctrine does not employ argument to prove its principles, which are articles of faith, but proceeds from them to demonstrate something else, as the Apostle argues from the resurrection of Christ to that of others.  The inferior sciences, indeed, do not prove their first principles or argue with those who deny them, ut leave them to e proved by a higher science, while metaphysics, which is the highest among them, only disputes with those who deny its first principles if the adversary grant something; if he grant nothing it cannot argue with him, but can still solve his objections.  In like manner, Sacred Doctrine, having no superior, disputes with those who deny its principles provided the adversary grant something, and proceeds to argue from one article of faith against those who impugn others, as in the case of heretics.  If, however, the adversary believe none of those things which are revealed, it is no longer possible to prove the articles of faith by reason alone, but only to solve objections brought against a faith which is rooted in infallible truth.  For as it is impossible that what is contrary to truth can be proved to be true, the arguments brought against faith can be answered.

I t benefits Sacred Doctrine to express things Spiritual and Divine by the use of corporeal metaphors, for God provides for every creature according to his nature, and men are led naturally by means of things sensible to knowledge which is purely intellectual.  moreover, the Sacred Scriptures are for the instruction of all men, and the uncultured apprehend intellectual things more easily under corporeal similitudes.

And this Doctrine has several meanings under one letter.  There is the literal, which is also called the historical sense, and the spiritual, when the things expressed in the words mean something else.  The spiritual sense is threefold; for as the Old Law is the sign of the New, and the new Law is the figure of the future glory, there follows from this the allegorical sense.  As they signify what makes up eternal glory, they give the anagogical sense.  Since God is the author of this Doctrine, and he comprehends all things in the mind at once, it is most fitting that the one literal sense should contain many meanings.

On the Spirituality Page, keeping with the passion theme, I’ve posted links to THE GOSPEL OF LENT AND THE PASSION OF CHRIST, a series of meditations on the gospel texts used in the pre -Vatican II lectionary.  Another work is entitled  WE PREACH CHRIST CRUCIFIED, meditations for boys.  Bing an Aquinas dilettante, I’ve also linked to a work called LENT WITH ST THOMAS AQUINAS, compiled by Father Philip Hughes.  Finally, I’ve linked to a work called CROWN OF GLORY by Alban Goodier, S.J., whose cause for beatification is currently underway.

This word day is to be taken in a four-fold sense–”The day is at hand;” the day of mercy; the day of grace; the day of justice; the day of glory. The sun that makes this a fourfold day, whose advent holy Church now celebrates. The day of mercy is the birthday of the Lord, in which the Sun of righteousness arises upon us; or more truly, He who made that day so glorious. The day of grace is the time of grace; the day of justice is the day of judgment; the day of glory is the day of eternity. Joel speaks of the first: “In that day the mountains shall drop down new wine, and the hills shall flow with milk” (Jl 3:18). Concerning the second, Paul writes, “Behold, now is the day of salvation” (2 Cor 6:2). Of the third sense we read, “The day of wrath, that day of tribulation” (Wis). Concerning the fourth, “But it shall be one day which shall be known to the Lord-not day, nor night; but it shall come to pass that at evening time it shall be light” (Zech 14:7. One day in thy courts is better than a thousand” (Ps 139:10). The birthday of the Lord draws near, that devoutly the day of mercy may be celebrated and honored; the day of grace that it may be received; the day of judgment that it may be feared; the day of glory that it may be attained. The Church celebrates the first, “for the Lord is at hand” (Phil 4:5). “For my salvation is near to come, and righteousness is near to be revealed’ (Is 56:1). On account of the second (day), “behold, now is the accepted time; behold, now is the day of salvation” (2 Cor 6:2). On account of the third day, “Behold, the judge standeth before the door” (James 5:9). On account of the fourth day, “Behold, I come quickly, and My reward is with Me to give to every man according as his work shall be” (Rev 12:12).

We ought to celebrate the birthday of the Lord, the day of mercy, with mercy and truth. Christ came to us in these two ways (of mercy and truth), and so we ought to go to him. “All the paths of the Lord are mercy and truth” (Ps 25:10). (We ought) to celebrate the day of grace with purity and humility, for these two graces make acceptable grace. concerning purity, Proverbs 22:11 states, “He that loves pureness of heart, for the grace of his lips, the King shall be his friend.” Concerning mercy, we read in James 4:6, “God resists the proud, but gives grace to the humble.” The Church celebrates the day of judgment with meditation and fear. As St Jerome says, “Whether is eat or drink, that voice seems ever to resound in my ears, “rise up, you dead, and come to judgment.’” On the contrary, it is said of the wicked, “evil men understand not judgment” (Prov 28:5). We ought to run and hasten to meet the day of glory with righteousness. Let us labor, therefore, to enter into that rest” (Heb 4:11). To four Christian virtues the apostle exhorts us in this Epistle (to the romans). To mercy and to truth in the words, “Let us put on the amour of light.” For the arms of of light are mercy and truth; for mercy is the shield by which we are defended from the enemy, and truth is the power by which we overcome all things. Concerning mercy we read, “Shut up alms in your store-houses, and they shall deliver you from all affliction. They shall fight for you against your enemies better than a mighty shield and a strong spear” (Eccl 29:12-13). Concerning courage, we read, , “Truth is great, and will prevail; it is great, and stronger than all things; the whole earth invokes the truth, and it blesses heaven itself; it moves all work, and they tremble because of it, and there is no iniquity in it. A wicked banquet, a wicked king, wicked women, all wicked sons of men, and all their wicked works, and truth is not in the, and they shall perish in their iniquity, and the truth shall remain” (Eccl 3:4). The epistle further exhorts us to purity and humility, “Not in chambering and wantonness, not in strife and envy” (vs 13). Chambering and wantonness are acts of riot which make impurity. Strife and envying proceed from pride. In prohibiting immodesty it exhorts us to humility. In the words, “let us walk honestly, as in the day,” it awakens us to reflection upon and to fear of the judgment; that is, that we should live that is is meet to live in the day of judgment. A man is in the judgment by thinking upon the judgment; he live honestly by thinking upon the judgment. It exhorts us to judgment and to dispatch-”Now it is high time to awaken out of sleep;” and, therefore, by hastening from the sleep of sin, to arise to the fulfilling of justice; and the reason is given why a man should do this: “For now is our salvation nearer than when we believed;” to which salvation we may be led by Jesus Christ Our Lord.

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Article 1 of the Summa looks at the question: Whether, besides philosophy, any further doctrine is required?

This is treated in the first paragraph of the Comp:

• It is necessary for the salvation of man that, besides the natural sciences, there should exist some doctrine received by revelation which transcends reason. Moreover, that which is discoverable about God by human reason could be known only by a few, and that after much time, and not without a large admixture of error. It was good, therefore, for man to be taught by means of a doctrine divinely revealed; for salvation, which is in God, depends upon the knowledge of the truth.

St Thomas deals with this subject more fully in chapters 2, 3 & 4 of the Summa Contra Gentes. For more, see the full text of THE CATHOLIC ENCYCLOPEDIA article on revelation, especially the first 3 sections on pages 1-3. You can also profitably consult chapter 1 of the Catechism of the Catholic Church, as well as chapter 2.