Hence it is said: Not by the works of justice that we have done, but according to his own mercy he hath saved us (Tit. iii, 5): It is not of him that willeth, nor of him that runneth, but of God that showeth mercy (Rom. ix, 16): because man needs must be forestalled by the divine assistance for purposes both of willing well and doing well. As the victory is attributed to the general, which is won by the labour of the soldiers, so such expressions as the above are not to be taken as exclusive of the free choice of the will, according to the misconstruction which some have put upon them, as though man were not master of his own acts, interior and exterior, but they show that man is under God. Again it is said: Turn us, O Lord, to thee, and we shall be turned (Lament. v, 21): which shows that our turning, or conversion, is anticipated by the aid of God converting us. Still we read, as spoken in the person of God: Turn ye to me, and I will turn to you (Zach. i, 3); not that the work of God in us does not go before our conversion; but the meaning is that the conversion, whereby we turn to God, is aided also by His subsequent aid, strengthening it to arrive to effect, and securing it that it may reach its due term.
Hereby is excluded the error of the Pelagians, who said that the divine assistance is given us in consideration of our deservings; and that, while the beginning of our justification is of ourselves, the consummation of it is of God.
3.149 : That the Divine Assistance does not compel a Man to Virtue
3.151 : That the aforesaid Assistance is called 'Grace,' and what is the meaning of 'Grace constituting a State of Grace'