The charity of Christ is the source in us of all our merits before God. Grace, by uniting us to Christ in active love, ensures the supernatural quality of our acts and consequently their merit before God and before men. The saints have always had a lively awareness that their merits were pure grace.
After earth's exile, I hope to go and enjoy you in the fatherland, but I do not want to lay up merits for heaven. I want to work for your love alone.... In the evening of this life, I shall appear before you with empty hands, for I do not ask you, Lord, to count my works. All our justice is blemished in your eyes. I wish, then, to be clothed in your own justice and to receive from your love the eternal possession of yourself.
Source: Catechism of the Catholic Church (Vatican.va)