Answer
Binitarianism is the belief that the one true God exists as two Persons (the Father and the Son). Binitarianism is distinguished from Trinitarianism (God exists as three Persons) and Unitarianism (God exists as only one Person). It is also distinguished from bitheism (the belief in two gods). Binitarianism has never been a popular view of God and is held by a small number of groups today.
Binitarianism teaches that the Holy Spirit is actually just another name for Jesus—more to the point, that Jesus is simply the Spirit incarnate. The claim is based on the misreading of a passage in the non-canonical “The Shepherd of Hermas” and on Romans 8:9, which says, “You, however, are not in the realm of the flesh but are in the realm of the Spirit, if indeed the Spirit of God lives in you. And if anyone does not have the Spirit of Christ, they do not belong to Christ” (cf. 1 Peter 1:10–11). Here, the “Spirit of God” seems to be equated with the “Spirit of Christ”; if the “Spirit of Christ” is simply another name for Jesus, as Binitarianism contends, then there is no third Person of the Godhead.
A better way to understand Romans 8:9 is that the Holy Spirit is called the “Spirit of Christ” because He is sent by Christ (John 15:26), He testifies of Christ (John 15:26), He comes in the name of Christ (John 14:26), and He seals believers in Christ (Ephesians 1:13). The fact that the Son sends the Spirit from the Father shows that all three Persons of the Trinity are distinct.
The Trinity is a great mystery, and even the most learned Bible scholars cannot adequately explain it. However, the Bible, specifically the New Testament, teaches that the one true God exists as three Persons (Father, Son, and Holy Spirit). The Bible also teaches that it was the Son who became incarnate, not the Father or the Spirit. Thus, Binitarianism is not biblical.
Below is the best symbol for the Trinity we are aware of (click to expand):
