ABC News

With Republican Mitt Romney moving closer to becoming the first Mormon presidential nominee, I asked Pastor Rick Warren whether he believes Mormons are Christians - a contested issue among evangelicals.

TAPPER: Mitt Romney will almost certainly be the Republican nominee, and if that happens, as it looks like it will, he'll the first member of the Church of Jesus Christ of Latter-Day Saints to be the nominee. A lot of evangelicals have been talking about whether or not Mormons are Christians. Are Mormons Christians?

WARREN: Well, the key sticking point for evangelicals and actually for many is the issue of the Trinity.  Orthodox Christians, Catholic Christians, Protestant Christians, evangelical Christians and Pentecostal Christians all believe in the Trinity; that's the historic doctrine of the church, that God is three-in-one. Not three gods; one God in Father, Son and Holy Spirit.

WARREN: Mormonism denies that. That's a sticking point for a lot of Catholic Christians, evangelical Christians, Pentecostal Christians, because they don't - they don't believe that. Now they'll use the same terminology, but they don't believe in the historic doctrine of the Trinity. And people have tried to make it other issues. But that's really one of the fundamental differences.