Question: Are “Christians” who don’t understand or believe the traditional doctrine of the Trinity really Christians?
Answer: That’s a good question. I take it you’re asking whether they have eternal life. The traditional answer would be “No.” I believe the Roman Catholic, Eastern Orthodox, and historic Protestant Churches would say you have to believe the Trinity to get final salvation. For example, the Athanasian Creed says, “Anyone who does not keep it whole and unbroken will doubtless perish eternally.” That creed includes several doctrines, but chiefly, the Trinity. It is saying that if you don’t believe the Trinity as defined by the creeds, then you’ll go to hell. So, traditionally, people would say that if you don’t believe the Trinity, then you aren’t really a “Christian” in the sense of being saved.
However, I would not give the traditional answer. Instead, I’d be careful to make a distinction between what you must know to be born again, and the more advanced truths you need to grow to spiritual maturity.
The only condition I see in Scripture to be born-again is to believe in Jesus for everlasting life (John 3:16, 36; 5:24; 6:47, etc). Although I believe in the Trinity as traditionally formulated, I don’t see anywhere in Scripture that says believing in the Trinity is necessary to be born again. (Technically, there is no passage that directly teaches the Trinity at all!)
However, I would take it as an “advanced” truth that’s important (I don’t want to say it is necessary) for discipleship and maturing in the faith. Eventually, the more you study Scripture, and the more you meditate on Jesus’ life and actions, you’ll begin to see that Jesus was more than a man. As you start to compare verses like John 1:3-4 and Col 1:16, you’ll realize that Jesus created the world. And who does that? Not merely a man. Then you’ll look at other verses, such as John 1:1 and John 20:28 that explicitly say Jesus is God. You’ll start to wonder about the theophanies in the Old Testament. Maybe it will start to dawn on you that the Father, Jesus, and the Holy Spirit are more intimately connected than you realized before.
I will say that I think that some level of recognition of Jesus’ deity is necessary to become spiritually mature. But I’m not willing to say you must believe in the Trinity as defined by Nicaea and Chalcedon for that to happen, because there is no Scripture that says so. I don’t want to add to Scripture. However, I do think the traditional definition of the Trinity best explains the Biblical evidence about Christ’s deity and His relationship to the Father and the Spirit (e.g., that God is tres personae, una substantia in the Latin West, or mia ousia, treis hypostaseis in the Greek East—i.e., that God is three Persons in one Substance). Typically, if you depart from the traditional theology of the Trinity, you will have some very odd and aberrant views of Christ’s deity. And that may lead you astray in your spiritual life.
But getting to that traditional understanding of the Trinity is not obvious and it’s not easy. And Biblically speaking, I don’t think it’s necessary to have eternal life.