Preaching the "belief" that mormonism is the "true and living church" is, of course their belief...however, that statement is ALSO saying that all other faiths are false. Preaching that there was a "great apostasy that caused God's Gospel to be removed from the earth" is also their belief...and is ALSO saying that all other faiths were false and God removed his Gospel.
Focusing on what they believe that is a direct contradiction to Christian belief is repugnant.
True, but what about those who teach that Christ is an essential part of salvation? They can do that a number of ways— either focusing on the good that belief in Chist has brought to their lives, all the way to condemning others as heathen savages. That approach went over well in the 19th century, but I suspect many Christian churches (outside fringe groups like Westboro Baptist Church or the United Church of YHWH) have chosen a different approach.