All the Scriptures were written for this: because the man understood that GOD loves him and, understanding it, will burn inside of love toward Him

GOD is One. No one, not even GOD can create another God.

Pange, lingua, gloriosi
Corporis mysterium
Sanguinisque pretiosi,
Quem in mundi pretium
Fructus ventris generosi
Rex effudit gentium.
Sing, my tongue, the Savior’s glory,
Of His Flesh the mystery sing;
Of the Blood, all price exceeding,
Shed by our immortal King.
Pange, Lingua (hymn for Vespers on the Feast of Corpus Christi), stanza 1

Down in adoration falling,
Lo! the sacred Host we hail;
Lo! o’er ancient forms departing,
Newer rites of grace prevail;
Faith for all defects supplying,
Where the feeble senses fail.
Pange, Lingua, stanza 5 (Tantum Ergo)

Thus Angels’ Bread is made
The Bread of man today:
The Living Bread from Heaven
With figures doth away:
O wondrous gift indeed!
The poor and lowly may
Upon their Lord and Master feed.
Sacris Solemniis Juncta Sint Gaudia (Matins hymn for Corpus Christi), stanza 6 (Panis Angelicus)

O saving Victim, opening wide
The gate of heaven to man below,
Our foes press on from every side,
Thine aid supply, Thy strength bestow.
Verbum Supernum Prodiens (hymn for Lauds on Corpus Christi), stanza 5 (O Salutaris Hostia)

A hymn is the praise of God with song; a song is the exultation of the mind dwelling on eternal things, bursting forth in the voice.
Commentary on the Psalms , Introduction

Anything done against faith or conscience is sinful.
Commentary on Romans, cap 14, I 3

For creation is not a change, but that dependence of the created existence on the principle from which it is instituted, and thus is of the genus of relation; whence nothing prohibits it being in the created as in the subject. Creation is thus said to be a kind of change, according to the way of understanding, insofar as our intellect accepts one and the same thing as not existing before and afterwards existing.
Summa Contra Gentiles II, 18.2 (see also Summa Theologica I, q. 45, art. 3 ad 2)

The perfection of the effect demonstrates the perfection of the cause, for a greater power brings about a more perfect effect. But God is the most perfect agent. Therefore, things created by Him obtain perfection from Him. So, to detract from the perfection of creatures is to detract from the perfection of divine power.
Summa Contra Gentiles, III,69,15

Natural inclinations are present in things from God, who moves all things. So it is impossible for the natural inclinations of a species to be toward evil in itself. But there is in all perfect animals a natural inclination toward carnal union. Therefore it is impossible for carnal union to be evil in itself.
Summa Contra Gentiles, III,126,3

The highest perfection of human life consists in the mind of man being detached from care, for the sake of God.
Summa Contra Gentiles, III,130,3

Three things are necessary for the salvation of man: to know what he ought to believe; to know what he ought to desire; and to know what he ought to do.
Two Precepts of Charity (1273)

Reason in man is rather like God in the world.
Opuscule II, De Regno

Charity, by which God and neighbor are loved, is the most perfect friendship.
Quaestiones disputatae: De caritate (ca. 1270)

It must be said that charity can, in no way, exist along with mortal sin.
Quaestiones disputatae: De caritate (ca. 1270)

We can open our hearts to God, but only with Divine help.
Quaestiones de veritate disputatae q 24, art. 15, ad 2

It is on account neither of God’s weakness nor ignorance that evil comes into the world, but rather it is due to the order of his wisdom and the greatness of his goodness that diverse grades of goodness occur in things, many of which would be lacking if no evil were permitted. Indeed, the good of patience would not exist without the evil of persecution; nor the good of preservation of life in a lion if not for the evil of the destruction of the animals on which it lives.
De Potentia (On Power) q. 3, art. 6, ad 4

If… the motion of the earth were circular, it would be violent and contrary to nature, and could not be eternal, since … nothing violent is eternal …. It follows, therefore, that the earth is not moved with a circular motion.
Commentaria in libros Aristotelis de caelo et mundo

The greatness of the human being consists in this: that it is capable of the universe.
De Veritate (On Truth) q. 1, art. 2, ad 4

If man of himself could in a perfect manner know all things visible and invisible, it would indeed be foolish to believe what he does not see. But our manner of knowing is so weak that no philosopher could perfectly investigate the nature of even one little fly.
Sermon on the Apostles’ Creed, prologue (trans. Joseph B. Collins)

Suppose a person entering a house were to feel heat on the porch, and going further, were to feel the heat increasing, the more they penetrated within. Doubtless, such a person would believe there was a fire in the house, even though they did not see the fire that must be causing all this heat. A similar thing will happen to anyone who considers this world in detail: one will observe that all things are arranged according to their degrees of beauty and excellence, and that the nearer they are to God, the more beautiful and better they are.
Sermon on the Apostles’ Creed, 13-14

All that I have written seems like straw compared to what has now been revealed to me.
Remarks on being requested to resume writing, after a mystical experience while saying mass on or around 6 December 1273, as quoted in A Taste of Water : Christianity through Taoist-Buddhist Eyes (1990) by Chwen Jiuan Agnes Lee and Thomas G. Hand
Variant translations:
All that I have written seems like straw to me.
As quoted in The Thought of Thomas Aquinas (1993), by Brian Davies, p. 9
Everything I have written seems like straw by comparison with what I have seen and what has been revealed to me.
As quoted in Sacred Games : A History of Christian Worship (1997) by Bernhard Lang, p. 323