"Therefore the distinguishing doctrines on which I do insist in all my writings and in all my preaching will lie in a very narrow compass. You sum them all up in Perceptible Inspiration. For this I earnestly contend; and so do all who are called Methodist preachers. But be pleased to observe what we mean thereby. We mean that inspiration of God's Holy Spirit whereby He fills us with righteousness, peace, and joy, with love to Him and to all mankind. And we believe it cannot be, in the nature of things, that a man should be filled with this peace and joy and love by the inspiration of the Holy Spirit without perceiving it as clearly as he does the light of the sun.
This is (so far as I understand them) the main doctrine of the Methodists. This is the substance of what we all preach. And I will still believe none is a true Christian till he experiences it; and, consequently, ‘that people at all hazards must be convinced of this -- yea, though that conviction at first unhinge them ever so much, though it should in a manner distract them for a season. For it is better that they should be perplexed and terrified now than that they should sleep on and awake in hell.’
I do not, therefore, I will not, shift the question; though I know many who desire I should. I know the proposition I have to prove, and I will not move an hair’s breadth from it. It is this: ‘No man can be a true Christian without such an inspiration of the Holy Ghost as fills his heart with peace and joy and love, which he who perceives not has it not.’ This is the point for which alone I contend; and this I take to be the very foundation of Christianity. . . .
It would doubtless be wrong to insist thus on these things if they were ‘not necessary to final salvation’; but we believe they are, unless in the case of invincible ignorance. In this case, undoubtedly many thousands are saved who never heard of these doctrines; and I am inclined to think this was our own case, both at Oxford and for some time after. Yet I doubt not but, had we been called hence, God would first, by this inspiration of His Spirit, have wrought in our hearts that holy love without which none can enter into glory."
~ John Wesley, December 30, 1745 letter to John Smith (the pen name of an anonymous writer thought to be Dr. Thomas Secker who would later become Archbishop of Canterbury)