The Greek word for God is Theos. It is from this word we get many English words referring to God, as we shall see. For example, polytheism is the belief in many gods.
This belief was common before the coming of Christianity. In a Christian context polytheism was essentially the faith that the apostles had to confront when they began to evangelize the non-Jews. It has to be said, though, that by the time of Christ many of the philosophically-minded, educated pagans were moving beyond polytheism to a belief in one God, somehow beyond and above the many lesser gods and goddesses.
We now move on to monotheism, the belief in one God. This is common to Christianity, as well as Judaism and Islam. In addition, many Hindus are monotheistic. Of course, we as Christians believe in the Trinity, one God in three persons, but this doesn’t negate our belief that God is one. This doctrine is not based on philosophical consideration, but on the apostles personal experience of God made flesh in the person of Jesus Christ. In other words, the apostleals, as good Jews, believed in God, but their experience of Jesus Christ, especially after His resurrection, convinced them that Jesus was God.
Atheism, of course, is the denial of the existence of God. Although there probably have always been atheists, their numbers are growing in our society and they are becoming more militant. One cannot prove the existence of God with arguments that would convert an atheist to Christianity, but we can certainly be witnesses to our faith and give reasons for our faith, but we should never argue. If we have friends or relatives who are atheists, the best thing we can do is pray for them.
Hinduism, of course, is basically polytheistic. However, there are many forms of Hinduism that are monotheistic. In addition, some forms of Hinduism believe in one, divine reality of which everything else, the cosmos, human beings, the various gods and goddesses, are simply manifestations this.
Buddhism does not have any concept of God at all. However, it is better to say that Buddhism is non-theistic rather than atheistic. Buddhists themselves realize that atheism in its modern form is a reaction to Christianity, whereas Buddhism grew in a different context entirely.
If we look at the Judaism from which Christianity has arisen, the belief in one God is expressed more succinctly in the most basic creed of Judaism “Hear, O Israel: The Lord our God is one Lord….” (Deut 6:4)
Pious Jews recite this daily in public and private prayer. It is out of this creed with its uncompromising, monotheism that the Christian creed has developed. Although Jews disagree with the Christian belief in the Holy Trinity, it is a development of this monotheism, not a denial if it.