In one sense you do not have to believe in the Trinity to be a Christian, but in another sense you do. Let me explain. The Bible does not tell us that we must believe in the Trinity in order to become saved; that is, to become a Christian. On the other hand, true Christians will end up believing in the Trinity because it is the proper teaching concerning God’s nature that has been revealed to us in Scripture. So, though someone may not understand the Trinity when he or she becomes a Christian, eventually he will end up believing in it because he’s a Christian.
The doctrine of the Trinity is the proper biblical teaching concerning the nature of God. It is one of the defining elements of the Christian faith. The Trinity, like the deity of Christ (John 8:24; John 1:1, 14; Colossians 2:9), Christ’s physical resurrection (1 Corinthians 15:14, 17), and justification by faith alone in Christ alone (Romans 3:28; 4:1-5; 5:1; Galatians 2:21), are among the essential doctrines of the Christian faith. To deny any of these is to deny what makes Christianity Christian. But, I want to make it clear that we believe these things because they are true and because the Holy Spirit who indwells Christians bears witness of truth (John 14:26; 15:26).
It is not believing in the Trinity that makes us Christian. Rather, it is being a Christian (and being indwelt by the Holy Spirit) that enables us to believe in the Trinity. So, in one sense it’s not a requirement to affirm the doctrines the Trinity in order to become saved. However, the true Christian will not deny the doctrine of the Trinity because the Holy Spirit will bear witness of truth (John 15:26) in the Trinity as true. This would mean that anyone who claims to be a Christian but openly and continually rejects the doctrine of the Trinity, is probably not truly saved.
