How should I go about evangelizing my atheist, homosexual friend back into the Catholic Church and to being heterosexual?
First off, be a true friend, sticking with your friend even if he does not come around quickly. Winning him back to Jesus Christ and his Church may take years, so realize you need to persevere as a friend and ask the Lord to help you do so. Never condone his morally problematic actions, but don’t abandon him either. Jesus never abandons a lost sheep (see John 10:1-21, Luke 15:3-7), although at the end of his life a person may reject Christ.
In the process, make sure you share the Faith in a charitable manner (see 1 Peter 3:15), and don’t always make that the focus of your discussions lest your friend pull away from you. There’s a maxim about the fraternal correction of adults that a priest friend of mine often shares and that is based on the wisdom of St. Thomas Aquinas: It should be done privately (so as not to embarrass), charitably (in emulation of Our Lord), and rarely (given that frequent correction will likely be counterproductive since you’re speaking to an adult and not a child).
Regarding the issues at hand, your goal should not necessarily be to persuade him to date women (or men, if your friend is a woman). It should be to help him live a chaste life, regardless of whether he ever dates or marries a woman. In that regard, we highly recommend Courage, a faithful Catholic apostolate serving those dealing with same-sex attraction. Even if there is not a Courage chapter in your area, your friend could establish solidarity with Courage members from afar.
There may be a good counselor or others locally who can accompany your friend in his walk with the Lord. In addition, here are some online links to our free resources on homosexuality. And here also are some resources for purchase.
Regarding atheism, here are some links to our free resources on atheism. And here also are some resources for purchase.