This is a rather lengthy read, so I suggest you scroll down to “
Conditions of Merit.” It states:
The necessity of the third condition, i.e., of the influence of actual grace, is clear from the fact that every act meriting heaven must evidently be
supernatural just as heaven itself is supernatural, and that consequently it cannot be performed without the help of prevenient and assisting grace, which is necessary even for the just.
In the parable of the vine Christ expressly declares the “abiding in him” a necessary condition for “bearing fruit”: “He that abideth in me, and I in him, the same beareth much fruit” (
John 15:5); and this constant union with Christ is effected only by
sanctifying grace.