So saving grace, converting grace, for Augustine, is God's giving us a sovereign joy in God that triumphs over all other joys and therefore sways the will. The will is free to move toward whatever it delights in most fully, but it is not within the power of our will to determine what that sovereign joy will be.
John PiperWhen I believe in Jesus, I am united to Christ. Therefore, what he did and achieved becomes mine by this union through faith alone. His righteous life is imputed to me. What Christ achieved is counted as mine.
John PiperThe suffering of sickness and the suffering of persecution have this in common: they are both intended by Satan for the destruction of our faith, and governed by God for the purifying of our faith... Christ sovereignly accomplishes His loving, purifying purpose, by overruling Satan's destructive attempts. Satan is always aiming to destroy our faith; but Christ magnifies His power in weakness.
John Piper