Their zeal and evangelism definitely affected Europe as well the United States. With churches being started in a variety places because of their work and efforts. John's Standard Sermons are drawn upon by the Methodist, Nazarene and Wesleyan churches for much of their doctrinal standards. These men came out of the culture of the Church of England, and had respect for its roots. However, saw grace and holiness in a more definitive or personal manner to be expressed.
Their influence should be considered insightful and relevant in the development of evangelical thought in the church. As they truly were tremendously powerful in their influencing the evangelical Protestant movement.
Wesley's 52 Standard sermons and notes can be found online for study. With a nice layout to really get into the meat of the sermons. I also have found a recent book by Adam Hamilton entitled " Revival " an invaluable tool in understanding the Wesley's background, history and ministry as a whole. It is also a very relevant book for anyone trying to understand the concept of revival to.
I challenge you to consider a study of the Wesley's its quite illuminating and thought provoking.
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Here are the links to those study materials.
Wesley's 52 Standard sermons
Revival at amazon