Aquinas argues that we can conclude this without recourse to revelation.
[I]t must also be shown that in the creation of things God did not work of necessity, as though He brought things into being as a debt of justice (SCG II-28, 1; p. 79ff).God did not owe anything to his creatures when he made us.
As Aristotle points out, justice involves a relationship to another, to whom it renders what is due. But, for the universal production of things, nothing is presupposed to which anything may be due. It follows that the universal production of things could not result from a debt of justice (II-28, 2).That seems pretty obvious once it's brought to your attention: you can't "owe" anything to what doesn't exist, because there is nothing "there" to which something could be due.
There is also, more specifically, the question of being in debt to another.
Furthermore, no one owes anything to another except because he depends on him in some way, or receives something either from him or from someone else, on whose account he is indebted to that other person; a son is a debtor to his father, because he receives being from him; a master to his servant, because he receives from him the services he requires; and every man is a debtor to his neighbor, on God’s account, from whom we have received all good things. God, however, depends on nothing, nor does He stand in need of anything that He may receive from another, as things previously said make perfectly clear. Hence, it was from no debt of justice that God brought things into being (II-28, 4).There is no possible way that God can become indebted to us, since (in the first place) he lacks nothing, and there is nothing that we could provide to him that he in any way requires.
And these conclusions are consistent with the Bible. "Who hath first given to him, and recompense shall be made him? For of Him, and by Him, and in Him, are all things" (Rom. 13:35-36). "Who has given me before that I should repay him? All things that are under heaven are mine" (Job 41:2).
These things being the case, it ought to be obvious that God does not owe us anything. More to the point, we cannot earn salvation; hence those who claim that we hope to do so are mistaken (or, if they know the truth of what we believe, then they slander us). "Every good gift and every perfect gift is from above, coming down from the Father of Lights" (James 1:17). But that which is merited is not a gift, but rather something that is due. Hence we do not merit salvation, but rather it is a gift from God.