“Informed Latter-day Saints do not argue that historic Christianity lost all truth or became completely corrupt. The orthodox churches may have lost the ‘fullness’ of the gospel, but they did not lose all of it nor even most of it. Many Evangelicals caricature or overstate the actual LDS view, which is that the orthodox churches are incomplete rather than corrupt. It is their postbiblical creeds that are identified in Joseph Smith’s first vision as an ‘abomination,’ but certainly not their individual members or their members’ biblical beliefs.”

Stephen E. Robinson
Craig L. Blomberg and Stephen E. Robinson, How Wide the Divide? A Mormon and an Evangelical in Conversation (Downers Grove, IL: InterVarsity Press, 1997)
http://www.mormonnewsroom.org/article/authority-in-the-church