Is Christianity truly a monotheistic religion?

[Immortal Goat]

I have read some very interesting theories about the Christian religions. If you believe that you are following a monotheistic religion, then you cannot believe in a devil that is a rival to your god. If it is a TRULY monotheistic religion, there can be NO rival to God, for that being would be a god themselves. If you insist on believing in demons and a Satan, then it is required that they are not rivals, but simply servants to a god that, far from being purely good, is simply neutral.

In the Bible, it is read that "I form the light and create darkness; I make peace and create evil. I the Lord do all these things." (Isaiah 45:7) God's sun and rain pour on the good and evil alike and make both weeds and wheat grow.

So, does that sound like the typical Cristian belief in God? Didn't think so.