It is serious, it is a matter of life or death - eternally because no False religion leads to salvation. Mormonism is a false teaching poising as Christian & that is utter nonsense , it presents a False Christ, and a False gospel just as the Lord Jesus warned followers to not be deceived by.
From Mormon website:
There teaching about Adam and Eve.
"Because Adam and Eve chose to follow God the Father, we are all able to come to earth, as they did, to receive a body, learn and grow, and become more like God and Jesus Christ, so that , we can return to live with Them again someday."
This is serious spiritual deception. That one little sentence describing Adam and Eve contains 5 blatant Lies. The deceived deceiver Joseph Smith, lead by demonic inspiration is what the Bible warns is a false teacher. Joseph Smith began his own false cult trying to ride upon the teachings of Christianity. Smith blasphemes & corrupts the Christian teachings by adding to the Bible scripture , by subtracting from the Bible scripture, and he presents a False Christ, substitutes a false gospel & false salvation which leads people away from the True Gospel of Jesus Christ and Salvation. This means his false teaching is leading people to hell.