It has been pointed out by many when they figure out the ramifications of monergism: if the only way we are saved is that God enables us to have faith by the Holy Spirit, then isn’t it unfair that not everyone is given such grace?
Reading through Augustine’s On the Gift of Perseverance, I found a nice, canned answer to the question:
It is therefore settled that God’s grace is not given according to the deserts of the recipients, but according to the good pleasure of His will, to the praise and glory of His own grace; so that he who glories may by no means glory in himself, but in the Lord, who gives to those men to whom He will, because He is merciful, what if, however, He does not give, He is righteous: and He does not give to whom He will not, that He may make known the riches of His glory to the vessels of mercy.
For by giving to some what they do not deserve, He has certainly willed that His grace should be gratuitous, and thus genuine grace; by not giving to all, He has shown what all deserve. Good in His goodness to some, righteous in the punishment of others; both good in respect of all, because it is good when that which is due is rendered, and righteous in respect of all, since that which is not due is given without wrong to any one (Chapter 28).