“Justification by faith only” is a truth that needs interpretation. The principle of sola fide is not rightly understood till it is seen as anchored in the broader principle of sola gratia. What is the source and status of faith? Is it the God-given means whereby the God-given justification is received, or is it a condition of justification which it is left to man to fulfill? Is it a part of God’s gift of salvation, or is it man’s own contribution to salvation? Is our salvation wholly of God, or does it ultimately depend on something we do for ourselves? Those who say the latter (as the Arminians later did) thereby deny man’s utter helplessness in sin, and affirm that a form of semi-Pelagianism is true after all. It is no wonder, then, that later Reformed theology condemned Arminianism as being in principle a return to Rome (because in effect it turned faith into a meritorious work) and a betrayal of the Reformation (because it denied the sovereignty of God in saving sinners, which was the deepest religious and theological principle of the Reformers’ thought). Arminianism was, indeed, in Reformed eyes a renunciation of New Testament Christianity in favour of New Testament Judaism; for to rely on oneself for faith is no different in principle from relying on oneself for works, and the one is as un-Christian and anti-Christian as the other. In the light of what Luther says to Erasmus, there is no doubt that he would have endorsed this judgment.”
J.I. Packer, quoted in Barrett, Matthew M. (2013-07-29). Salvation by Grace: The Case for Effectual Calling and Regeneration (pp. 13-14). P&R Publishing. Kindle Edition.