St. Teresa says this in Interior Castle, about the soul in mortal sin:
While in a state like this the soul will find profit in nothing, and hence, being as it is in mortal sin, none of the good works it may do will be of any avail to win it glory; for they will not have their origin in that First Principle, which is God, through Whom alone our virtue is true virtue. (Emphasis added)
These are cautionary words on two counts, it seems to me. In the first place, it declares the truth that the Church has always taught about human merit: it comes from God, and consequently it is in that sense not ours. This is why St. Augustine once said that when God crowns our merits, He crowns His own gifts to us. The point is that many Catholics and most critics of the Catholic Church get this completely wrong. We think that we can win brownie points with God by doing this or that good thing, and we become proud of what virtuous people we are. Ahem. That is not what the Church teaches, as St. Teresa, Doctor of the Church, reminds us. We Catholics and our critics would do well to remember the facts of the matter.
And the second point is practically the same as the first. We think wrongly about this subject, and the consequence is that we wind up behaving wrongly. We may presume upon God’s forgiving grace, thinking that we can clean up behind ourselves all by ourselves. We commit grave sins and think that they are counterbalanced by the five dollar bill we gave the beggar and by holding the door open for the person behind us or by going to worship services, or whatever other good thing we think will make up for our sin. Wrong. As St. Teresa reminds us, the soul in mortal sin cannot save itself. None of its good works amount to a hill of beans before God. Forgiveness and salvation are found in Christ alone.