It all comes down to faith.
1 From this it becomes clear that men are bound to the right faith through divine law.
2 Indeed, just as the origin of bodily love lies in the vision accomplished through the bodily eye, so also the beginning of spiritual love ought to lie in the intellectual vision of an object of spiritual love. Now, we cannot possess the vision of God, as an object of spiritual vision, in this life except through faith, because it exceeds the power of natural reason, and particularly because our happiness consists in the enjoyment of Him. Therefore, we must be led to the right faith by the divine law.
3 Again, the divine law orders man for this purpose, that he may be entirely subject to God. But, just as man is subject to God as far as will is concerned, through loving, so is he subject to God as far as intellect is concerned, through believing; not, of course, by believing anything that is false, for no falsity can be proposed to man by God Who is truth. Consequently, he who believes something false does not believe in God. Therefore, men are ordered to the right faith by the divine law.
4 Besides, whoever is in error regarding something that is of the essence of a thing does not know that thing.
Thus, if someone understood irrational animal with the notion that it is a man, he would not know man.
Now, it would be a different matter if he erred concerning one of man’s accidents. However, in the case of composite beings, the person who is in error concerning one of their essential principles does know the thing, in a relative way, though he does not know it in an unqualified sense. For instance, he who thinks that man is an irrational animal knows him according to his genus. But this cannot happen in reference to simple beings; instead, any error at all completely excludes knowledge of the being.
Now, God is most simple. So, whoever is in error concerning God does not know God, just as the man who thinks that God is a body does not know God at all, but grasps something else in place of God. However, the way in which a thing is known determines the way in which it is loved and desired. Therefore, he who is in error about God can neither love God nor desire Him as an end. So, since the divine law intends this result, that man love and desire God, man must be bound by divine law to bold a right faith concerning God.
Notes About man’s nature vs. accidents: all men have two legs by nature, some have only one by accident. That’s a joke, but a good one. Some men have longer, some shorter legs; leg length is an accident, in metaphysical terms.
5 Moreover, false opinion holds the same place in regard to objects of the intellect that vice opposed to virtue has in regard to moral matters, “for truth is the good of the intellect.” But it is the function of divine law to prohibit vices. Therefore, it also pertains to it to exclude false opinions about God and matters concerned with God.
Notes It should be obvious enough that to believe truths is better than to believe falsehoods. But how do you find truths about God? Stick around.
6 Thus it is said in Hebrews (11:6): “Without faith it is impossible to please God.” And in Exodus (20:2) before the other precepts of the law are given, right faith concerning God is put in first place; moreover, it is said: “Hear, O Israel: the Lord Your God is one” (Deut. 6:4).
7 Through this consideration we exclude the error of those who say that it makes no difference to the salvation of man whatever be the faith with which he serves God.