"Circumcision is circumcision of the heart, by the Spirit, not by the written code. Such a person’s praise is not from other people, but from God." (Romans 2:29, NIV)
(Notes: I went with a friend on my first-ever backpacking trip last week, so although last week was the 17th week of the year, I'm posting this a little late. In a couple of days, I'll post this week's sermon, Wesley's Sermon 18: The Marks of the New Birth. Also, in searching for an image to use with this post, I had the choice of searching for images about the heart or about circumcision. I hope I made the right choice.)
This sermon from Wesley touches on several things that are key to understanding him, without going into too much depth on any of them: the nature of faith, the relation of faith to works, the centrality of love, and Wesley's doctrine of assurance.
This sermon's structure is also pretty simple, as he breaks down the meaning of the circumcision of the heart with four virtues: humility, faith, hope, and love ("charity"). Then, after describing what each of these means, he lists ways that people can know if their hearts have been circumcised... If you have humility, faith, hope, and love, they have been. If not, they haven't.
Wesley also includes a great description of the ultimate characteristic of those with circumcised hearts as "having God as their chief and only end." Other traditions will describe this same characteristic as simplicity, purity of heart, or willing one thing.
If you would like to dig in further: