In Gen 20:13 Abraham tells Abimelech that "the Gods caused me to wander." The verb "caused," in Hebrew, is in the plural form to match "Gods."
In Psalms 82, the psalmist clearly writes about how God presided over the other Gods in a council he called.
There are various other mentions of multiple Gods as the early Hebrews had many of them and tried to consolidate over the years. The New Testament also obviously and clearly refers to multiple Gods. People stumble over themselves while bending over backwards to try and explain it away, but more than one God mentioned is more than one God no matter how much spin is placed on it. Here we have a classic case of begging the question: it is assumed that there is only one god, and this assumption is used to back up claims for one god. Looking at the evidence, we can see there are many Gods.
Let's see. God the Father sent his Son to die for our sins. God the Son prayed to God the Father in the Garden of Gethsemane to try to change God the Father's mind. God the Son cried out to God the Father, "Why have you forsaken me?" on the cross. God the Son was born after God the Holy Spirit impregnated Mary. God the Son said not to blaspheme God the Holy Spirit as it was the one unforgivable sin, unlike blaspheming him or God the Father.
If there ain't three of them, then Christians are worshiping a schizophrenic.
The question notes that we are to use the definition of "gods" that we use to conclude a religion is polytheistic.
We can see that the devil in Christianity is just as much a god as Hades is in roman based polytheism. That angels are as much a god as Mercury.
At the end of the day we must realize that while Christians don't care to be looked at as being polytheistic, a rose by any other name smells just as sweet. And Christianity smells like a polytheistic rose.
The problem Christianity had in its early days was PRECISELY one of polytheism. Early Christians (pre-Council of Nicaea) were divided on whether or not "Jesus" WAS god or just the son of god. It is actually where we get the term "an iota of difference"- the Greek terms for "like substance" (Jesus being the son of god) and "same substance" (Jesus and god are the same) only differed by the letter iota.
The problem was thus:
If Jesus was god, the atonement makes no sense. How can one sacrifice himself to himself to get around his own proclamations? How is that even a sacrifice? Why would "god" need to manufacture a middle man?
If Jesus was the son of god, Christians were breaking one of the Commandments by worshiping multiple gods- Jehovah and Jesus.
Ultimately the Council decided Jesus WAS god, because they could conjure up some theological double-talk to get around the atonement issue but obvious polytheism was a bigger problem- especially if they actually wanted to convert Jews.
If you look at the Lore behind the Bible in some of the books left out or thought to be unnecessary, you will see that the picture painted isn't as clearly Monotheistic. First, look at the book of Enoch. So much of it listing hierarchies of Angels and Demons. Even naming a lot of them. This dovetails quite nicely into what some would see Greek Polytheism as. Lets not forget the history of Tribal Judaism... Its started as a polytheist religion and slowly they began to fixate the system around their Sky God.
This is a very uninformed statement that the bible is polytheistic. It comes from the lack of understanding biblical text,history, and context. God takes many forms, but does that mean these 3 beings are completely different God's? Of course not! Ice and water are the same, just different forms. The same is true for the bible and Christianity.
Christians only believe in one God. This God is omnipotent, so he can take many forms. The Holy Trinity is still the same God, whose name is God. There's no reason to believe that Christianity is polytheism. (poly : multi - theo : god). It was nice to think about, though.
I assume you're referring to the "Holy Trinity", which are different manifestations of the same deity (i.E. God). Some Hindus argue that Hinduism is monotheistic - that all the different "gods" are different representations/manifestations (please correct me if this is wrong!). In the same way Christianity - as I understand it - uses each representation to represent different facets of the same God (e.G. Christ offers forgiveness, God offers judgement, the Holy Spirit offers nourishment). As an abstract idea I'm not sure I can offer a concrete argument that they MUST be simply three depictions of the same entity; rather I've tried to offer the concept that allows the three to exist while allowing the religion to remain monotheistic
There is no "we" and Christianity is certainly not a polytheistic religion. The other side offers no real argument. Their problem seems to be with religion in general and no attempt at serious study of the Trinity. Perhaps an easy way to help them understand the concept would to point out that like water, which can exist in three separate states (solid, liquid, and gas), it is still one...Water.
(Please pay attention to my capitalization, it makes a difference - God = Christian God whom we worship, god= noun defined here as any holy being) God is The Father, and He is the only God. His son (The Son) is Jesus Christ, and the Holy Ghost or Holy Spirit is what people "feel" when they experience the "presence" of God. There are three entities, but only one is God. Jesus is not a god, the Spirit is not a god, but rather the physical manifestation of God, and what people experience. The Spirit is difficult for me to explain, because it sounds like I'm saying It is God, but I am not.