Christianity is monotheistic - there is one God. But it is also trinitarian - three persons in the Godhead (three in one, "triple unity", trinity).
Obviously, understanding the fundamental nature of God is a hard problem. He is infinite and holy, while we are finite and not. But God has stooped down to our level to reveal Himself, and we must try to reconcile what has been revealed.
Attacks on the Trinity usually come in one of two forms:
- Jesus is not God (Arianism)
- God is not three "persons", but rather, three "modes", "roles" or "forms" (Sabellianism, or modalism)
- The Old Testament (Is 9:6) says the child born unto us will be called the "mighty God" and "everlasting Father".
- Sin is an offence, and only the one offended can forgive. When Jesus forgives sins, He is claiming to be God (the people at that time understood this, and sometimes took up stones against Him)
- In Hebrew the term "son", in the metaphorical sense, means to have the nature and attributes ("son of destruction" is a destructive person). We think of "son" as derivative (less than), but that is not the original meaning. "Son of God" is claiming to be God (again, the people at that time understood this, and took up stones).
- God is love, and love is a relationship (there must be some object for the love). Under orthodoxy, the three Persons love each other. Apart from that, it is hard to say what God's love is.
- We see all three Persons present at Jesus' baptism. (Also, there are two other occasions where the Father speaks from Heaven to Jesus on Earth).
- God is love - number four above.
- Jesus repeatedly prays to the Father. Is Jesus addressing Himself?
- We see the Trinity at work in many places. The Resurrection is attributed to all three Persons. Salvation is the work of all three.
Finally, it is good to note that no one would fabricate such a hard doctrine. The easy way would be to support modalism or Arianism (Arianists excercised a great deal of power for a time). But we cannot deny what the Bible teaches, God is Three in One.