St. Paul was referring to the Sacraments

If we look at the teachings of St. Paul, it is clear that he had the Sacraments in mind when teaching justification.

Let's begin at Romans 2:13, we see that St. Paul says:
Romans 2: 13(For not the hearers of the law are just before God, but the doers of the law shall be justified.

That simple statement, reminds me of RCIA. Converts to the Church are taught to keep the Commandments and then to request Baptism.

Doers of the Law shall be justified.

There were no chapters back when the Scriptures were written. Therefore, it is in this same stream of thought, that St. Paul continues and says that:
Romans 3:28
Therefore we conclude that a man is justified by faith without the deeds of the law.

The CCC says:
1216 "This bath is called enlightenment,....It is called gift because it is conferred on those who bring nothing of their own; ....

In the Sacraments, all we bring is our faith. Therefore the Church says:
1127 Celebrated worthily in faith, the sacraments confer the grace that they signify.....

This is clearly Sacramental teaching.

Sincerely,

De Maria