A few years back a friend and I were talking with an authority-type figure about how John MacArthur's Ashamed of the Gospel shredded the ecumenical idea and the church-growth movement. We were told in response that MacArthur had been one of the signees to the Evangelicals and Catholics Together (ECT) document and surely could not be relied upon to take a strong stand. By the grace of God and the power of Google, that allegation was quickly debunked. But I wondered how many people heard such a rumor and assumed it was true.
The fact that some folks have now been saying that John MacArthur has been going fuzzy on Mormonism is news to me. But just in case you have heard or eventually do hear such nonsense, read these three posts from Phil Johnson for the real story.