In the world of the early church, most non-Christians were polytheists. Many believed in the false gods of the Greeks and Romans, while others worshipped the idols of the ancient Near East. There were also polytheists who believed in cosmic powers, and some who worshiped the elements or other aspects of creation. Atheism — the belief that no gods exist — was rare.

One reason that belief in various gods was so common was that polytheism was often required by law. For example, in the Roman Empire, the government enforced the worship of the Roman gods. The Romans required this worship in order to gain the gods’ favor and protection for the Empire. But a more basic reason for the belief in various gods was the sinfulness of human beings.

The Bible indicates that humans are very prone to turn away from the true God to false gods. This has to do especially with the Bible’s doctrine of sin. Sin acts in such a way as to actually blind us even with regard to the truth of God as God has revealed it to us in creation. And so, left to ourselves we will in fact identify as “God” or identify as divine qualities those things that are not true of God. In other words, we will create gods of our own imagination as substitutes for the true God. [Dr. David Bauer]

Listen to the way Paul talked about this in Romans 1:20-23:

Since the creation of the world God's invisible qualities — his eternal power and divine nature — have been clearly seen, being understood from what has been made, so that men are without excuse. For although they knew God, they neither glorified him as God nor gave thanks to him, but their thinking became futile and their foolish hearts were darkened... [They] exchanged the glory of the immortal God for images made to look like mortal man and birds and animals and reptiles.

In our sin, human beings do not naturally acknowledge the true God. Instead, we attribute his work to other sources.

With this picture of polytheism in mind, we are ready to explore monotheism, the belief that only one God exists.

Now, we need to point out that not all monotheistic religions worship the same god. Judaism, Christianity and Islam each worship only one god. And more than this, they all identify this one god as the God of Abraham, at least in name. But the concepts they attach to the name “God of Abraham” are very different.

Consider Judaism. Judaism denies the Trinitarian God that the Bible reveals. In fact, Jewish theologians deny each person of the Trinity. They reject Jesus as Lord and God incarnate. They deny that the Holy Spirit is a divine person. And by rejecting Jesus and the Holy Spirit, they deny the Father who sent them. As Jesus said himself in Luke 10:16:

He who rejects me rejects him who sent me.

And when we consider Islam, it is even clearer that their concept of God contradicts the Bible.

Islam’s conception of God does indeed contradict the Bible, and one of the most significant ways in which it contradicts the Bible is in the assertion that God is absolutely one and there is no community of being within him. In Christian theology there is an absolute fidelity to monotheism. That Christian doctrine of the Trinity — that the one God eternally exists in three persons, Father, Son, and Holy Spirit, there is a real and substantial fellowship between persons in that one true God — that is a radically different conception of God than Islam has. [Dr. J. Ligon Duncan III]

There are many religions besides Christianity in the world today. But the Bible insists that only the Christian God is truly divine; only the Christian God will judge the world; and only the Christian God has the power to save us.