Christianity is monotheist.

A common suggestion by apoligists of Islam is that Christianity is not truly monotheistic. I would like to disprove this assertion.

Jesus makes it very clear (Mark 12:28-29) that there is only one Lord and God:

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28And one of the scribes came, and having heard them reasoning together, and perceiving that he had answered them well, asked him, Which is the first commandment of all?

29And Jesus answered him, The first of all the commandments is, Hear, O Israel; The Lord our God is one Lord
At the same time He claims (John 10:30) that:

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30 I and my Father are one.

So Jesus establishes that within the one God there actually exists a realationship between Father and Son. Added to this the Holy Spirit, which Christ also speaks of as another aspect of God, we have relationships between Father, Son and Spirit.

Christianity has always maintained that Mark 12:29 means that we are monotheistic. There is ONLY one God. Anyone who says otherwise is not Christian.

However, Jesus' claim that He is one with the Father means that there exist relationships within God, between Himself and the Father, but without there being "gods" plural.

Christian theologians, for obvious reasons, found this very hard to explain. It took many years for the human mind to begin to understand this "mystery" of the Faith. In the Church a wonderful analogy was divised to explain.

In ancient greek and roman theatre, there were very few trained actors, and no real costumes: the same actor would play several parts by simply wearing a different mask whilst in the role of each character. In latin, mask is persona, plural personae. In english we still use "persona" to denote a personality or character, rather than an independent human being, as "person is commonly used today."

So by using the analogy of a single actor with several personae the Church set out to explain how God could be Father, Son and Holy Spirit. The characters in a play remain distinct but with no real change in the actor; there remains only one actor.

In the same way, in God there may be three "persons", each a different and distinct persona of the Almighty (Trinitas), but they are all simply one God in essence (Unitas).

In the Latin (western) Church, this doctrine was best explained by the Athanasian Creed:

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Whosoever will be saved, before all things it is necessary that he hold the catholic faith. Which faith except everyone do keep whole and undefiled, without doubt he shall perish everlastingly. And the catholic faith is this: That we worship one God in Trinity, and Trinity in Unity, neither confounding the persons, nor dividing the substance.

For there is one Person of the Father, another of the Son, and another of the Holy Spirit. But the godhead of the Father, of the Son, and of the Holy Spirit, is all one, the glory equal, the majesty co-eternal.

Such as the Father is, such is the Son, and such is the Holy Spirit. The Father uncreated, the Son uncreated, and the Holy Spirit uncreated. The Father incomprehensible, the Son incomprehensible, and the Holy Spirit incomprehensible.

The Father eternal, the Son eternal, and the Holy Spirit eternal. And yet they are not three eternals, but one Eternal.

As also there are not three incomprehensibles, nor three uncreated, but one Uncreated, and one Incomprehensible. So likewise the Father is Almighty, the Son Almighty, and the Holy Spirit Almighty. And yet they are not three almighties, but one Almighty.

So the Father is God, the Son is God, and the Holy Spirit is God. And yet they are not three gods, but one God.

So likewise the Father is Lord, the Son Lord, and the Holy Spirit Lord. And yet not three lords, but one Lord.

For as we are compelled by the Christian verity to acknowledge each Person by Himself to be both God and Lord, so we are also forbidden by the catholic religion to say that there are three gods or three lords.

The Father is made of none, neither created, nor begotten. The Son is of the Father alone, not made, nor created, but begotten. The Holy Spirit is of the Father, neither made, nor created, nor begotten, but proceeding.

So there is one Father, not three fathers; one Son, not three sons; one Holy Spirit, not three holy spirits.

And in the Trinity none is before or after another; none is greater or less than another, but all three Persons are co-eternal together and co-equal. So that in all things, as is aforesaid, the Unity in Trinity and the Trinity in Unity is to be worshipped.

He therefore that will be saved is must think thus of the Trinity.

Furthermore, it is necessary to everlasting salvation that he also believe rightly the Incarnation of our Lord Jesus Christ. For the right faith is, that we believe and confess, that our Lord Jesus Christ, the Son of God, is God and man; God, of the substance of the Father, begotten before the worlds; and man of the substance of his mother, born in the world; perfect God and perfect man, of a rational soul and human flesh subsisting. Equal to the Father, as touching His godhead; and inferior to the Father, as touching His manhood; who, although He is God and man, yet he is not two, but one Christ; one, not by conversion of the godhead into flesh but by taking of the manhood into God; one altogether; not by confusion of substance, but by unity of person. For as the rational soul and flesh is one man, so God and man is one Christ; who suffered for our salvation, descended into hell, rose again the third day from the dead. He ascended into heaven, He sits at the right hand of the Father, God Almighty, from whence He will come to judge the quick and the dead. At His coming all men will rise again with their bodies and shall give account for their own works. And they that have done good shall go into life everlasting; and they that have done evil into everlasting fire.

This is the catholic faith, which except a man believe faithfully, he cannot be saved.