John Wesley was jealous. His closest friends and even his brother had all recently had these passionate conversions of the heart and he was just doping around believing in God with his brain but not really feeling it. So he reluctantly went to a little service on Aldersgate Street, and during a reading of Martin Luther’s Preface to Romans, John said, “I felt my heart strangely warmed.” That was coupled with a sense of assurance in what Christ had done for him.
Methodists everywhere refer to this as this great conversion experience for John, even though John didn’t really see it that way even like a week later. At first he was all like, “I wasn’t even a Christian before!” And that really offended a lot of people who thought he had always been a better Christian than them. He did put a lot of emphasis on felt assurance throughout his ministry, but over time he lightened up on that being a requirement for salvation.