Nobody converts to being 'non denominational'. That merely means that someone is Protestant but doesn't identify with any particular Protestant denomination. It's common in the US for Protestants to do this. Trump was raised Presbyterian but often attended Episcopal (Anglican) services. He was married in the Episcopal Church and often attended this Episcopal parish in Palm Beach: https://www.bbts.org George Bush Jr also left the church he was raised in (Episcopal) to attend evangelical services in other Protestant churches. In Bush's case, however, there was a serious reason for doing so as the Episcopal Church is liturgical and generally socially liberal, so it didn't give him the sort of evangelical, biblical fundamentalism he sought after his 'born again' religious conversion which was helped along by the late Billy Graham.Similarly, Trump converted while in the office to appease his Evangelical base, going from Presbetarian to non-denominational. It might seem like a small detail, but it was very significant politically. Mainstream Protestants and Catholics are miles away from Evangelicals on a lot of issues.