@galena @Godcast @alyx Yes friend Mormons are not Christians because they do not have a valid baptism. The simplest definition for a Christian is a person who has been validly baptized and has not apostatized from the faith. You may also consider catechumens as Christians if you wish, although they are not fully there yet. An early concept was the baptism of blood and baptism of desire, which could apply to catechumens who either are martyred or otherwise die before being baptized normally.
For a baptism to be valid, it must be done in the form that Jesus mentioned. He told us to baptize in the name of the Father, and of the Son, and of the Holy Spirit. Water must also be used either by triple immersion, pouring, or sprinkling. We must also not actively reject any of the core fundamentals of the faith. In the early Church someone would have to learn the apostles creed and affirm it before they entered into the Church. The Nicene creed was used later as it had greater clarity.
So going back to the Mormons, they use the proper form of baptism, and use the water properly, but reject the core fundamentals by believing that God the Father, Jesus, and the Holy Spirit are three separate gods rather than one God in three persons.
Another example of an invalid baptism is some Pentecostal groups. They do not reject the core teachings of the faith, and use the water properly, but will baptize in Jesus' name only. This makes their baptisms invalid.
Here is a list which you may use for reference. Catholic falls into the valid group, although it is not noted here because this is a list made by Catholics specifically for non-Catholic groups. Thank you friend.