Sexagesima Week. The worship due to God. The meditations and readings for Lent.From St. Thomas Aquinas. Translated by Fr Philip Hughes.

Thou shalt not have strange gods before me-Exod.XX.3
We are forbidden to worship any but the one God, and there are five things which show the prohibition to be reasonable.
1. God's dignity. If this is disregarded we insult God. To all dignity is due proper reverence. And we call a traitor who refuses to do the King due reverence. This is what some men do with respect to God. They changed the glory of the incorruptible God into the likeness of the image of a corruptible man, and of birds, and of fourfooted beasts and of creeping things, says St. Paul (Romans I.23). And all this is the most serious of all offences against God.

2. God's bountifulness. Every good thing we possess comes from God. It is in fact part of God's dignity that he is the maker and giver of all good things. When thou openest all things shall be filled with good (Ps. CIII.28). You are therefore ungrateful beyond measure if you do not recognise that good you have is his gift. Nay, you make yourself another god as truly as the children of Israel. Delivered from Egypt, made themselves an idol. This is to be like the harlot of whom the prophet writes, I will go after my lovers that give me my bread and my water, my wool and my flax, my oil and my drink (Osee II.5). This sin is also committed by those who place another in preference to asking it from God. Blessed is the man whose trust is in the name of the Lord (Ps.XXXIX.5), and St. Paul marvels at the Galatians, But now, after that you have known God, or are rather known by God, how turn you again to the weak and needy elements, which you desire to serve again? (Gal. IV.9).

3. Our promises. We have renounced the devil and pledged our fidelity to God alone. This pledge we must keep unbroken. A man making void the law of Moses, dieth without any mercy, under two or three witnesses. How much more do you think he deserveth worse punishment, who hath trodden underfoot the Son of God, and hath esteemed the blood of the testament unclean, by which he was sanctified, and hath offered an affront to the Spirit of Grace? (Heb.X.28,29).
The woman that hath an husband, whilst her husband liveth she shall be called an adulteress, if she be with another man (Rom. VII.3), and such deserves to be burned. Woe to the sinner, to whoever enters the land by a double way, to those who limp one foot on each side of the division.

4. The weight of the devil's yoke. You shall serve strange gods day and night, says the Prophet. Which shall not give you any rest (Jer. XVI.13).For the devil does not rest content with one sin, but, the first sin committed, strives all the more to induce us to another. Whoever commits sin is the slave of sin. Hence it is not an easy thing to find one's way out of sin. St. Gregory says, "The sin which is not lightened by penance, soon, by its very weight, drags us to further sin."
It is the very contrary that is characteristic of God's dominion over us. For God's commands are not burdersome. My yoke is sweet and my burden is light (Matt.
XI.30). A man is accounted as doing enough if he does for God as much as he has done for sin. St. Paul, for example, says, As you have yielded your members to serve uncleanness and iniquity, unto iniquity; so now yield your members to serve justice, unto sanctification (Rom. VI.19). But of the slaves of the devil the Scripture says, We wearied ourselves in the way of iniquity and destruction, and have walked through hard ways (Wis. V.7), and also, They have laboured to commit iniquity (Jer. IX.5).

5. The immensity of our reward. No law promises so great a recompense as that which are promised in the law of Christ. To the Saracens are offered rivers of milk and honey, to the Jews the promised land. But to Christians angelic glory. They shall be as the angels of God in heaven (Matt. XXII.30). Thinking on this St. Peter says, in the Gospel, Lord to whom shall we go? Thou hast the words of eternal life (John VI. 69).
(In Decalog.XII)