In my opinion, the design of the parable is simply this: So long as the pilgrimage of the Church in this world continues, bad men and hypocrites will mingle in it with those who are good and upright, that the children of God may be armed with patience and, in the midst of offenses which are fitted to disturb them, may preserve unbroken stedfastness (sic) of faith.
this cannot be understood simply of the persons of men, as if by creation God sowed good men and the devil sowed bad men. I advert to this, because the present passage has been abused by the Manicheans, for the purpose of lending support to their notion of two principles. But we know that whatever sin exists, either in the devil or in men, is nothing else than the corruption of the whole nature. As it is not by creation that God makes his elect, who have been tainted with original sin, to become a good seed, but by regenerating them through the grace of his Spirit; so wicked men are not created by the devil, but, having been created by God, are corrupted by the devil, and thrown into the Lord’s field, in order to corrupt the pure seed.