St. Thomas on Justification - God gives that which he rewards

In discussing the fact that mercy and justice accompany one another in God's works, Aquinas appeals to the example of Christ's treatment of Mary Magdalene as a response to the objection that they are seen separately.

Certain works are attributed to justice, and certain others to mercy, because in some justice appears more forcibly and in others mercy. Even in the damnation of the reprobate mercy is seen, which, though it does not totally remit, yet somewhat alleviates, in punishing short of what is deserved.

In the justification of the ungodly, justice is seen, when God remits sins on account of love, though He Himself has mercifully infused that love. So we read of Magdalen: "Many sins are forgiven her, because she hath loved much" (Luke 7:47). [ST I, Q21, A4, ad 1; emphasis added]

Although she has loved, there is no merit in her for her own sake because the Lord has given that which he rewards. But if God gives that which he rewards, then there is no way that we can merit anything from him on our own apart from his gift. Therefore we cannot merit justification, and consequently those who say we believe otherwise are misinformed.