Christianity is a monotheistic religion, which means that Christians believe there is only one God. Within that singular Godhead, Christians believe in the Trinity: God the Father, God the Son (Jesus Christ), and God the Holy Spirit. These are understood as three distinct persons, but united in one being.
The Bible, specifically the Old Testament, contains numerous commandments against worshipping other gods. For example, the first of the Ten Commandments is "You shall have no other gods before me" (Exodus 20:3). This is a foundational principle in Judaism, from which Christianity emerged.
Jesus himself affirmed this in the New Testament. When tempted by Satan in the wilderness, Jesus said, "Away from me, Satan! For it is written: ‘Worship the Lord your God, and serve him only.’" (Matthew 4:10).
Therefore, the core belief of Christianity is that worship should be directed solely to the one God, who is understood and revealed through Jesus Christ. To worship other gods would be a violation of this fundamental principle.