1 Corinthians 3:6

I have planted, Apollos watered; but God gave the increase. I was the first to sow the seeds of the faith at Corinth, and then Apollos coming after me helped it forward (Act 18:26). But it was God who gave the inner life and strength of grace for growth and maturity in Christian faith and virtue: this belongs to God alone. Cf. Augustine (in Joan. Tr. 5).

God gives to plants their increase, not, as rustics suppose, by directly adding some special daily power of growth, but by bestowing upon and preserving to the nature itself of the seed or the root a vigorous power of growth. In other words, He is continually bestowing it and preserving it, and co-operating with it: for the Divine work of preservation is nothing but a continuation of the primal creative power. He does this by ordering and tempering according to His counsel the rain, heat, and winds, and other things needed by the fruits of the ground, so that, as these are tempered, the fruit is larger or smaller. So it is in the sowing of the Word of God, and in its growth, perfecting and harvest in the minds of men.

It appears from this (1.) that outward preaching, calling, examples, and miracles are not alone sufficient for the conversion and the beginning of the spiritual life, or for its further growth. (2.) That, though all alike hear the same word of preaching, yet some profit little, some profit much by it, viz., those whom God works upon by a special inward calling, and whose hearts He touches to change their lives, or to continue to rise to higher things. Hence, both those who preach and those who hear profit most who earnestly beseech God for this inward influence.