A Mormon asked me this question a number of years ago, and through the years here at church, I’ve asked a number of people this question, and I wanted to get your opinion. Can you become a Christian if you deny the Trinity?
I would answer, "No." If you don’t believe in the Trinity, then you don’t understand who God is. You may say the word “God” but you don’t understand His nature. Second, you couldn’t possibly understand who Christ is—that He is God in human flesh. The Incarnation of Christ is an essential component of the biblical gospel, as John 1:1-14 and many other biblical passages make clear. To deny the Trinity is to deny the Incarnation. And to deny the Incarnation is to wrongly understand the true gospel.
In saying that, I realize that such an answer is going to not only impact people that you may have witnessed to (like Mormons), but it also applies to some in the broader Pentecostal movement, called United Pentecostals or "Jesus-Only" Pentecostals. Such individuals hold to a kind of modalism, where God is sometimes in the mode of the Father or the mode of the Son or the mode of the Spirit, but He’s never all three at the same time. That too is a deficient and heretical view of the Trinity. It denies the distinct Personhood of the Father, the Son, and the Holy Spirit.
The same question sometimes arises about the virgin birth. I think it is possible for a person to become a Christian before learning about the details of the virgin birth, though that person would certainly assume that Jesus Christ must have had a unique birth since He is both God and man. But, if someone knows about the virgin birth and says, “I deny the virgin birth,” then he is simultaneously denying the deity of Christ, and also the Trinity. Such a person betrays the fact that they do not understand the gospel, and therefore cannot have truly been saved.