But it didn't get lost, did it? We still have the gospels, which contain Jesus' words. The question is whether or not God would have allowed the truth to be corrupted, and let Christianity go off track for almost two millennia.
Matthew 16:18b provides a stronger case against the Mormon (and some Protestant) positions:
NASB: "[...]upon this rock I will build my church, and the gates of the netherworld [many translations have "hell"] shall not prevail against it."