1 Corinthians 3:7

So then neither is he that planteth anything, neither he that watereth, but God giveth the increase. The husbandman who plants and waters does hardly anything when compared with God; for he works from without only, and whatever he does he receives it from God, and works as His instrument. But God works within directly as the chief agent, and supplies the power of vigorous growth. For action is assigned to the chief agent, and especially to the first cause. So S. Thomas and Theophylact; S. Augustine (in i. Ep. S. John. Tr. 7) says beautifully: " Outward ministries are helps and warnings, but He that teacheth the heart has His throne in heaven. These words which we address to another from without are to him as the husbandman to the tree. For the husbandman acts upon the tree from without, by diligently watering and tending it, but He does not fashion its fruits." It is God that co-operates with the tree, and lends it the power of bringing forth fruit. In the same way the words of the preacher do but little, for they sound from without only. But it is God who co-operates with them within, and by His grace illuminates and converts the soul.