“Historically, it is a simple matter of fact that Martin Luther and John Calvin, and, for that matter, Ulrich Zwingli, Martin Bucer, and all the leading Protestant theologians of the first epoch of the Reformation, stood on precisely the same ground here. On other points, they had their differences; but in asserting the helplessness of man in sin, and the sovereignty of God in grace, they were entirely at one. To all of them, these doctrines were the very life-blood of the Christian faith.
The doctrine of justification by faith was important to them because it safeguarded the principle of sovereign grace; but it actually expressed for them only one aspect of this principle, and that not its deepest aspect. The sovereignty of grace found expression in their thinking at a profounder level still, in the doctrine of monergistic regeneration – the doctrine, that is, that the faith which receives Christ for justification is itself the free gift of a sovereign God, bestowed by spiritual regeneration in the act of effectual calling.
To the Reformers, the crucial question was not simply, whether God justifies believers without works of law. It was the broader question, whether sinners are wholly helpless in their sin, and whether God is to be thought of as saving them by free, unconditional, invincible grace, not only justifying them for Christ’s sake when they come to faith, but also raising them from the death of sin by His quickening Spirit in order to bring them to faith. Here was the crucial issue: whether God is the author, not merely of justification, but also of faith; whether, in the last analysis, Christianity is a religion of utter reliance on God for salvation and all things necessary to it, or of self-reliance and self-effort.” J.I. Packer, Introduction to Luther’s Bondage of the Will