Mormons teach that the Father, Son, and Holy Spirit are three Gods, and that we too can become Gods in our own right someday.
You may reply to them, for instance, with James 2:19, “You believe that God is one. You do well. Even the demons believe that and tremble,” but Mormons will have some explanation for that New Testament passage which fits their theology.
Oneness Pentecostals teach that the Father, Son, and Holy Spirit are not three persons but three manifestations of one divine person, God.
You may ask them who Jesus is praying to in Matthew 26:39 when he says in the Garden of Gethsemane, “My Father, if it is possible, let this cup pass from me; yet, not as I will, but as you will,” yet Oneness Pentecostals will offer some answer for why Jesus is not praying to another person.
Jehovah’s Witnesses teach that Jesus is not God, not divine, but God’s first and greatest creature, and that the Holy Spirit is not a person but the active force of God the Father in the world.
You may answer with the beginning and end of the Gospel of John: with John’s prologue where we see “the Word was with God, and the Word was God. … And the Word became flesh,” and the gospel’s climax, John 20:28, “Thomas answered and said to [Jesus,] ‘My Lord and my God!'” However, Jehovah’s Witnesses will surely have some reply for these verses.
In my personal experience, advocates of Mormon polytheism, Oneness Pentecostal modalism, or Jehovah’s Witnesses Arianism-esque theology have all been sincere, friendly, rational, and not unintelligent people. They were all well-versed in the Bible, regarded it as God’s infallible Word, and used it to support their beliefs. They all proudly claimed the name of “Christian.” However, the undeniable fact that their theologies contradict each other proves that these admirable personal traits are not enough to guarantee a true understanding of the Christian Faith. The problem is that there seems to be more than one possible internally-coherent interpretation of the Bible. Just as texts out of context can suggest several defensible, though incorrect, meanings; interpreting biblical texts outside the context of Christ’s one, holy, catholic, and apostolic Church leads to many errors.
Last week, two very nice Jehovah’s Witnesses visited me at my rectory and we spoke for a couple of hours. I believe we were debating whether one of Jesus’ “I Am” statements in John’s Gospel was a profession of his divinity when one of my guests remarked, “We can’t really be certain what he meant.” I replied to the effect, “You’re right!–If your opinion and my opinion are all we have to go on, if there’s no visible authority on earth with power from Jesus Christ to infallibly answer biblical questions, then we can never be certain our interpretations are true–since many sincere, reasonable, and even scholarly Christians firmly disagree. Without a clear, external teaching authority within the Church, we are left as sheep without a shepherd, and will inevitably scatter.” Most Christians revere the Holy Scriptures as God’s infallible Word, and this is right and good, but for some reason many of them reject the Catholic Church through which the Scriptures come.
Recall that Jesus wrote nothing in the Gospels (except perhaps something in the dust near the woman caught in adultery,) but Jesus did establish a Church. Through this Church the New Testament was written, collected, canonized, and revered. However, this process was certainly not completed in the first century AD. In the early Church there was much debate over which New Testament writings were inspired and should be included in the canon. The Shepherd of Hermas? The Book of Revelation? The Didache? The Letter to the Hebrews? The Epistle of Clement? Some early Church Fathers included works such as these in their lists of Bible books, while others left them out. It was the Catholic Church that ultimately canonized the New Testament books which all Christians acknowledge today.
One teaching shared by Mormons and Jehovah’s Witnesses is the belief that a Great Apostasy occurred in the early Church. The New Testament contains verses which warn about false teachers arising who will mislead many. A great deception, those religions say, happened soon after the death of the apostles and explains why the majority of self-professed Christians in history have held core doctrines widely different from their own. I would agree that false teachers and heresies arise in every age, but was there a Great Apostasy soon after the apostles that devastated Christ’s Church and caused his central teachings (like the true nature of God) to be discarded and forgotten?
All Christians will agree that Jesus was a wise man. Jesus was a wise man, indeed, who built his house on rock. Jesus declared to Simon, “‘I say to you, you are Peter [that is, you are “Rock” (in Greek)] and upon this rock I will build my church, and the gates of the netherworld shall not prevail against it.'” (Matthew 16:18) If Jesus was a wise man who built his house on rock, we can be assured that even though “the rain fell, the floods came, and the winds blew and buffeted the house–it did not collapse; [his Church] had been set solidly on rock.” (Matthew 7:24-25)
After building his Church on Peter for some forty years, did Jesus let it go into shambles and fail to repair it for about eighteen centuries, until Joseph Smith or The Watchtower came along? If so, Jesus really dropped the ball. If the Mormons or Jehovah’s Witnesses are right, then God managed to get all of the New Testament books infallibly written, correctly canonized, and faithfully preserved throughout millennia, but could not maintain the truth about himself in his Church on earth in the hearts and minds of believers much beyond the death of the apostles. More likely, our Lord Jesus Christ succeeded in preserving his teachings and the visible hierarchical authority he gave to his Church, from St. Peter (the first pope) and the apostles to Pope Francis and the bishops in communion with him today. A clear and necessary line of teaching authority runs though the centuries, through the laying of hands and apostolic succession.
You may encounter people who will present you with internally-consistent but very different interpretations of Scripture. Do not let your hearts be troubled. There are good reasons for everything we believe as Catholics. They may “know” the Bible, but we are blessed to know God’s Church from which the Bible comes. If you love Jesus Christ, love his Catholic Church. As even St. Joan of Arc, who personally experienced the complexities of the Church as a divine and human institution, said, “About Jesus Christ and the Church, I simply know they are just one thing and we shouldn’t complicate the matter.” If you love Jesus’ Church, you will love him well. Jesus Christ is risen and his Catholic Church, though ancient, has never died. Christ’s Church, the Bride he protects and for whom he laid down his life, is very much alive.