Let's keep priority first with our Lord. If a Mormon denies the Son, then the Father is not in him. Read 1John 2 (later half of the chapter is pertinent).
I've worked with many Mormons and they frequently have impecable morality. But morality is not to be confused with salvation. KOSMOS, or the World, is a system of making order out of chaos. There are many successful organizers in the world who have learned the social skills well prior to more technical skill and are very productive in society. Morality simply abides by legalism with respect of the will, marriage, family, and governments of others. Those are all divinely established institutions for believer and unbeliever alike. One can be very well meaning, moral, productive, yet not have a relationship with God. They might also have experienced supernatural phenomenon, that doesn't mean they have a spirit filled life with the Living God.
The Mormons I've met are very good people wrt human standards of morality, and where they practice false doctrine, I don't think they intend to deceive, so I wouldn't describe them as liars, but I don't find the same spirit filled life from the Holy Spirit in them. And how do I discern this? Again by the Holy Spirit, by faith, by continuous study of doctrine (advanced faith) and having that faith continually running through my thought processes as I continue to walk in Him.
Note that in these Scriptures incredibly valuable insight is available regarding the different persons of the Trinity, again one God, revealed in three different persons. That revelation provides far more than simply relating a few thoughts. It's an genius method of instruction and providing a living relationship between God and man all consistent with His plan and method of having relationship with us.
But the Mormons also read 1 John 2; they do not think their beliefs contradict 1 John 2. So, how do we determine whose interpretation is correct? If we just propose that both sides seek God, each side gets the 'burning in the bosom' in favor of his own interpretation.
And how do I discern this? Again by the Holy Spirit, by faith, by continuous study of doctrine (advanced faith) and having that faith continually running through my thought processes as I continue to walk in Him.
Of course, the Mormons say exactly the same thing. So this resolves nothing.
Any wholly subjective method of resolving theological disagreements is worthless. It is a recipe for complete fragmentation of the Church.
-A8