Even the mormons use the disunity argument to prove that they are the true church. Facts are:

1. Early Church was in disunity ( I Cor 1 )
2. True Church unity is spiritual in nature, not physical ( I Cor 12 ) Kingdom not of this world.
3. Disunity in another body of believers is a negative argument that does not really prove that the church pointing to the disunity in the others fold is by default the true church -- it is possible that both groups are false.
4. Joseph Smith was a false prophet.