Of course their (Mormon) baptism is useless. Just like your Catholic baptism.
Look. The Bible is quite clear and specific on baptism.
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Ephesians 4:5-6
King James Version (KJV)
5 One Lord, one faith, one baptism,
6 One God and Father of all, who is above all, and through all, and in you all.
**Just like your Catholic baptism.**
Whoever told you that? Some preacher who hates Catholics or some unnamed pamphlet?
Christ, founded the Catholic Church on the Apostles telling them in Matthew to “Go out to all the nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit. It’s right there in Scripture.
Why don’t you believe that Scripture?