The Church considers baptism as the first sacrament of the Church. To be a Christian is to be properly baptized.
Catechism of the Catholic Church:
818 " . . . . All who have been justified by faith in Baptism are incorporated into Christ; they therefore have a right to be called Christians, and with good reason are accepted as brothers in the Lord by the children of the Catholic Church."
871 "The Christian faithful are those who, inasmuch as they have been incorporated in Christ through Baptism, have been constituted as the people of God…
Every sacrament requires proper matter, form, and intention. Mormons use the proper matter (water) and use the correct wording, but their intention is not the same. Mormons do not share our faith in the Trinity as defined by the Council of Nicea nor do they share our understanding of original sin. Therefore even though they are using the Trinitarian formula they cannot mean and intend what the Church does through those words.