Ravenhill Sermon On Prayer and Evangelism

I am working on putting together a class on evangelism for my church. One of the classes will discuss one of the most forgotten parts of evangelism - prayer.

I would love to show a Ravenhill sermon as part of the class. I know that many of his sermons discuss prayer, but can anyone reccomend a specific one that focuses on the need of prayer in evangelism?

If you can't think of one by Ravehill, I am open to suggestions.

Thanks!