As tame as the Methodist Church is by modern standards, in the 1700s, it was considered radical and dangerous by many. Yet, it would spread like wildfire, and would emerge as the fourth largest denomination in all of Ireland. The first Methodists congregations in North America were planted by Irish immigrants.
The chief founder and theologian was John Wesley (1703-91), who with his brother Charles Wesley (1707-88) would gather Bible study groups within the Church of England. The Methodist Church would not emerge until after John Wesley’s death.
Although the Wesleyans were Arminian (Christ died for all people not just the elect) in their approach to theology, others were more Calvinistic-Methodist. Arminian theology as developed by John Wesley has come to be known as Wesleyan-Arminianism. However, Wesley refined Arminianism with a strong evangelical emphasis on justification by faith. He did departed from Classical Arminianism in the following areas, and this is where the Methodists were considered radical:
Atonement: Wesley sought a relationship between God’s love for people and God’s hatred of sin. To him it was not a legal demand for justice so much as an act of mediated reconciliation.
Possibility of apostasy: Wesley taught Christians could apostatize and lose their salvation. In Wesleyan-Arminianism, it’s not the sin committed that is the grounds for loosing salvation, but it is more related to experiences that are profound and prolonged. Wesley saw two areas in which a person could lose their salvation; unconfessed sin and the actual expression of apostasy. Wesley saw these as not permanent states, but the sinner could return.
Christian perfection: Wesley taught that Christians could attain a state of practical perfection wherein they lack all voluntary sin through the Holy Spirit. This was a state of perfect love and could happen in this life. This has also been termed entire sanctification. Perfect Christians did not mean they no longer violate the will of God, for involuntary transgressions remain. Christians remain subject to temptation and still have a need to pray for forgiveness and holiness. It is not an absolute perfection but a perfection of love. Some see this was Wesley’s greatest theological contribution to evangelicalism. He would outline his theology in A Plain Account of Christian Perfection (1777): http://gbgm-umc.org/umhistory/wesley/perfect.html Versions of this doctrine would birth the Wesleyan-Holiness and Pentecostal-Charismatic movement.
The Methodists would prove to be a highly successful evangelical movement in the British Isles. It caught the imaginations of the common person as it called for the individual to experience Jesus personally.
I would suggest the following books to understand Wesleyan-Methodist thought: Richard Clutterbuck, Handing on Christ: Recovering the Gift of Christian Doctrine (2009); Kenneth J. Collins, The Theology of John Wesley: Holy Love and the Shape of Grace (2007) and John Wesley: A Theological Journey (2003); Kenneth J. Collins and John H. Tyson, Conversion in the Wesleyan Tradition (2001); Randy L. Maddox, Responsible Grace: John Wesley’s Practical Theology (1994).