The most profound and exciting quest of the ages is the search for the ultimate meaning of the universe. Not many men are philosophers as they engage in this quest; but all pursue it, either with sharp awareness or with but a dim realization of their goal. Esthetic and social activity as well as theological or philosophical inquiry is significant indications of the universality and earnestness of this seeking after the final meaning of the world, which supports the life of men. Whether the result takes the form of sensuous pleasure, an ethical code, a metaphysical system, or a religious creed, the assumption is that a formula for living has been found which corresponds in some sense to what the universe finally means. Usually the clearest expression of this formula employs the vocabulary of religion, and particularly the word "God." The Old Testament, which the Jews follow as their holy book, tells them that the God is one. They have to believe on the oneness and purity of God.
And he said, tomorrow. And he said, be it according to thy word; that thou mayest know that there is none like unto the LORD our God. (Exodus 8:10)
Unto thee it was showed, that thou mightest know that the LORD he is God; there is none else beside him. (Deuteronomy 4:35)
Know therefore this day, and consider it in thine heart, that the LORD he is God in heaven above, and upon the earth beneath: there is none else. (Deuteronomy 4:39)
And he said, LORD God of Israel, there is no God like thee, in heaven above, or on earth beneath, who keepest covenant and mercy with thy servants that walk before thee with all their heart: (1 Kings 8:23)
Above verses were taken from the Old Testament (King James Version) idea of the unity of God is vigorously proclaimed, and it is made the foundation of faith and conduct. It is not accidental that the oneness of Israel's God and the requirement of absolute, unqualified love for him are announced in the same biblical passage.
Hear, O Israel: The LORD our God is one LORD (Deuteronomy 6:4)
and thou shalt love the LORD thy God with all thine heart, and with all thy soul, and with all thy might (Deuteronomy 6:5)
This oneness has both an intellectual and a religious basis. Reason seeks a unifying principle faith, a single object of supreme devotion. Thus man's philosophical quest for ultimate reality and his historical worship of the one God have contributed to a deepening certainty that God is one.
There are different Christian communities, which follow different versions of bible (new testament). All Christian community accepts the old testament (Kings James Version) in which the oneness of God is described.
And Jesus answered him, The first of all the commandments is, Hear, O Israel; The Lord our God is one Lord. (Mark 12:29)
And thou shall love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind, and with all thy strength (Mark 12:30)
Different communities of Christianity override the concepts given in that testament by newer version of Bible. There are two prominent communities, which believe in Trinity of God. One of them believes the Trinity to be the father, the son and the Holy Ghost.
And Jesus came and spake unto them, saying, all power is given unto me in heaven and in earth. (Mathew 28:18)
Go ye therefore, and teach all nations, baptizing them in the name of the Father and of the Son, and of the Holy Ghost: (Mathew 28:19)
The other community believe that the trinity to be the Father, the virgin Mary and the Son. The common concept of both the communities is the God is one however exists in three persons i.e. either father, son and Holy Spirit or father, Virgin Mary and Son.
Islam firmly gives the belief that the God is one.
In the name of God, Most Gracious, Most Merciful (Quran 112:0)
Proclaim, "He is the One and only GOD." The Absolute GOD. Never did He beget.