How did John Wesley understand original sin?

Great question! John Wesley addressed this directly, more than once. 

There were those in his day who thought the concept of original sin was outdated, or who simply ignored it. Wesley was not among them, as we see in his sermon “Original Sin,” part of our doctrinal standards, and his own edited version of John Taylor’s earlier treatise, The Doctrine of Original Sin: According to Scripture, Reason, and Experience.

In the sermon, Wesley makes clear he understands human nature has been so thoroughly corrupted that we have no possibility of any goodness at all in ourselves or in our patterns of thinking, apart from God’s grace intervening.

“But was there not good mingled with the evil? Was there not light intermixed with the darkness? No; none at all: ‘God saw that the whole imagination of the heart of man was only evil.’ It cannot indeed be denied, but many of them, perhaps all, had good motions put into their hearts; for the Spirit of God did then also 'strive with man,' if haply he might repent … But still ‘in his flesh dwelt no good thing;’ all his nature was purely evil: It was wholly consistent with itself, and unmixed with anything of an opposite nature.” (Sermon 44, Original Sin, I.4).

It was this pervasiveness of evil among and sinking deep into everyone, except Noah and perhaps some members of his family, that led God to destroy everyone else, with much of animal life as well, in the flood. And, he goes on, we are no better now as far as our natural state is concerned.

“The same account is given by all the Apostles, yea, by the whole tenor of the oracles of God. From all these we learn, concerning man in his natural state, unassisted by the grace of God, that ‘every imagination of the thoughts of his heart is’ still ‘evil, only evil,’ and that ‘continually.’” (Original Sin, II,1).

That overarching and underlying reality about us, every one of us, is why God has set about to save us through Jesus Christ in the power of the Holy Spirit.

Our Articles of Religion underscore this same point as they describe original sin as “the corruption of the nature of every man, that naturally is engendered of the offspring of Adam, whereby man is very far gone from original righteousness, and of his own nature inclined to evil, and that continually.” Left on our own, we will always choose the wrong way. The story of Adam and Eve names and cites the initial source of this corruption.

In Christ, we are given the opportunity to be restored into our true, uncorrupted human nature — to live into the life we were originally created to live. Wesley put it this way, “the great end of religion is to renew our hearts in the image of God, to repair that total loss of righteousness and true holiness which we sustained by the sin of our first parent” (Original Sin, III.5).  

Romans 5 explains, "as by one man's disobedience [Adam's] the many [meaning all who are born] were made sinners, so by the one man's obedience [Jesus] the many will be made righteous."

This is Paul's way of spelling out both the doctrine of sin and the doctrine of salvation. Christ can redeem all because his faithfulness to God in perfect love and obedience matches and exceeds the disobedience of one man, Adam.

Wesley concludes the sermon:

"Know your disease! Know your cure! Ye were born in sin: Therefore, 'ye must be born again,' born of God. By nature ye are wholly corrupted. By grace ye shall be wholly renewed. In Adam ye all died: In the second Adam, in Christ, ye all are made alive. 'You that were dead in sins hath he quickened:' He hath already given you a principle of life, even faith in him who loved you and gave himself for you! Now, 'go on from faith to faith,' until your whole sickness be healed; and all that 'mind be in you which was also in Christ Jesus!'"

Apart from God’s grace leading us to redemption, we have no hope of deliverance from the control of original sin over us. By God’s grace, made available to all in Jesus Christ and through discipleship to him with his church, we do.