John Wesley read Thomas Kempis' book, The Imitation of Christ, while in his youth - just before or during his time at Oxford. While he felt that Kempis was in error in refusing to enjoy anything in this world and in monastic isolation, he also recognized a call to holiness that was very absent from the overwhelmingly Christian population of England. The influence of Thomas Kempis' and other similar writings led Wesley to question his own faith and salvation and begin a search over a period of many years for what it meant to be a Christian, and what it meant to be saved.
At the end of his search, Wesley came to the conclusion that we are saved by grace alone, through faith alone. However, going against the grain of his time, he closely associated faith and holiness, proclaiming along with James that "faith without works is dead," - that faith without evidence of change of life and heart and growth toward holiness could not be saving faith, but rather the faith of the devil who "believes that there is a God and shudders." His rationale was this: if a person did not grow toward holiness (given the opportunity and proper teaching) it was either because that person did not know God's love and grace and thus could not respond to it, or knew God's love and grace yet failed to respond to it. In the first case, the person had not received salvation; in the second, the person had rejected it.
Sin, in Wesley's mind, is the opposite of faith. Willful sin (as opposed to sin of ignorance, poor judgment, etc) - whether sin of commission or omission - has no place in the Christian life. While an act of willful sin does not invalidate a Christian's faith an salvation, Wesley held it as a matter of utmost seriousness. This was in deep contrast to the practice of his day and, may I say, the practice of today in which forgiveness is emphasized, repentance is made easy, and the topic of sin is rarely touched upon. Few Christians are repulsed by the thought or idea of sin - that is how comfortable we have become with it.
John Wesley's insistence on holiness and the seriousness with which he viewed sin caused an uproar in the Church of England and in the larger society. The Methodist Society (which was still under the Church of England) went through great opposition, even persecution. Still, the movement grew. In his defense of the use of laity in ministry, he said, "Give me a hundred men who love nothing but God and fear nothing but sin...for such men will storm the very gates of hell." His statement proved true as the Methodist revival became the longest lasting revival since the early church, and became known as the Second Reformation.
Many non-Christians today are uncomfortable with the idea that it is possible to be saved merely by believing that Jesus died for our sins - and in a way, they are right. We are saved by grace alone, through faith alone, but faith without works is dead. We do not preach holiness as we should, and we forget that we are called to "be perfect, even as our Father in heaven is perfect." Oh that we would understand God's love, and because we fear anything that would separate us from it - however briefly - we would fear sin and strive for holiness in such a way that all around us would either mock us or wonder at the power of God in our lives and thus be drawn to Him.
