Catholics baptize in the name of the Father, Son, and Holy Spirit. So do Protestants. That’s why the Catholic Church accepts the validity of Protestant baptisms despite our different understanding of the nature of baptism.
What about Mormons? They also baptize in the name of the Father, Son, and Holy Spirit, but the Catholic Church has decreed their baptisms invalid.
Here’s why. (A full statement from the Vatican on this topic can be found here).
Every Sacrament must have the right form, matter, intent of the minister, and disposition of the recipient.
In baptism, the form is the formula you say when baptizing: “I baptize you in the name of the Father, the Son, and the Holy Spirit.” The matter is the water. For a baptism to be valid, the minister must pour water on the baptized — or immerse him or her in water — and say the correct words.
Most Protestants use the correct form and matter, which is why the Catholic Church accepts their baptisms.
Mormons use the water and seem to say the right words. So far, so good. However, their understanding of the Trinity is so vastly different than that of Catholics and most Protestants that it affects the form.
The Mormon understanding of God.
For Mormons, the Father is an exalted man with a physical body who lived on a planet somewhere in the cosmos. He also has a divine wife. The Son is not the Second Person of the Trinity. The Holy Spirit is not the spirit of the Father or the Son.
This led to the Vatican ruling that Mormon baptism doesn’t use the correct form: “There is not a true invocation of the Trinity because the Father, the Son, and the Holy Spirit, according to the Church of Jesus Christ of Latter-day Saints, are not the three persons in which subsists the one Godhead, but three gods who form one divinity.”
You must baptize with the same intent as the Church.
This doesn’t mean the baptizer needs a full understanding of the Church’s teaching on baptism. But he or she must at least have a willingness to share the Church’s intent. A Muslim, Jew, or Hindu can carry out a valid baptism in this way. However, these individuals in these rare cases must be administering baptism with the intent to do what both the Church and Christ want. The Council of Trent confirms this, stating that “Baptism administered by heretics in the name of the Father, the Son, and the Holy Spirit, with the intention of doing what the Catholic Church does is true Baptism.”
What we said above about the Trinity also applies here: The Mormon understanding of God means that Mormons don’t have the correct intent when they baptize.
The recipient of baptism must have the right disposition.
This means you must receive baptism in the way the Church intends, as a salvific act that cleanses you from sin. Most recipients of Mormon baptism are Mormons themselves. They likely believe the false doctrines of their religious leaders.
Therefore, anyone who was baptized as a Mormon should check with their priest about getting baptized in the Catholic Church.