Monergism

This truth that God justifies need to be underlined. We do not justify ourselves. Justification is not our apology nor is it the effect in us of a process of self-excusation. It is not even our confession nor the good feeling that may be induced in us by confession. Justification is not any religious exercise in which we engage however noble and good that religious exercise may be. If we are to understand and appropriate its grace we must turn our thoughts to the action of God in justifying the ungodly. At no point is the free grace of God more manifest than in his justifying act – “being justified freely by his grace through the redemption that is in Christ Jesus” (Rom. 3:24).
John Murray Redemption Accomplished and Applied (pg. 118)

Justification is God's act of remitting the sins of guilty men, and accounting them righteous, freely, by his grace, through faith in Christ, on the grounds, not of their own works, but of the representative lawkeeping and redemptive blood-shedding of the Lord Jesus Christ on their behalf.
J.I. Packer

“[God] reckons righteousness to them, not because he accounts them to have kept his law personally (which would be a false judgment), but because he accounts them to be united to one who kept it representatively (and that is a true judgment)” —J. I. Packer, “Justification,” in Evangelical Dictionary of Theology, ed. Walter A. Elwell [Grand Rapids, MI: Baker, 1984], p. 596.

The real reason why the doctrine of justification by grace alone through faith alone is unpopular is that it is grievously wounding to our pride.
John R.W. Stott