Christianity is a monotheistic religion, which means followers believe in one God. Christians believe God is the creator and sustainer of the universe (e.g. John 1:1-3, Col. 1:16-17, cf. Gen. 1:1). God is also all-powerful (or "omnipotent"), all-knowing (or "omniscient"), all-present (or "omnipresent"), and all-good (or "omnibenevolent"), according to the Bible. God's other attributes include: holiness, righteousness, immutability (i.e. unchanging), merciful, graceful, and eternal.
Additionally, Christians believe that God is comprised of three persons: (1) the Father, (2) the Son, Jesus Christ, and (3) the Holy Spirit. This doctrine is called the Trinity. This doctrine was articulated and affirmed at the Council of Nicea in 325 A.D. to counter Arianism, which was a teaching that argued that Jesus Christ wasn't actually God. The doctrine of the Trinity wasn't created at this church council, it was defended at it. More specifically, the council agreed that the Son is "of the same substance" as the Father, meaning he is eternal, and therefore any differences that exist between them occur within the divine unity.
Below you will find more in-depth discussions on God in the New Testament, God in the beliefs of the early church, God as masculine, and doctrinal statements about the doctrine of God from various Christian denominations.
God in the New Testament
The authors of the New Testament assumed the existence of the God of the Old Testament. They believed in Yahweh (also spelled "YHWH"), "the God of Abraham, Isaac and Jacob," whom the Jews worshipped as the one true God (Acts 13:32; Rom. 3:29, 4:3). Like the Old Testament, the New Testament teaches that there is only one God (Mark 12:29; Eph. 4:6; Jas. 2:19), who is pure spirit (John 4:24; 1 John 4:12), the creator of the world (1 Tim. 4:4; Heb. 3:4), holy and good (Rom. 3:4; Eph. 4:24; Rev. 4:8), all-powerful (Matt. 19:26; Mark 2:7, 10:18) and worthy of mankind's worship and love (Matt. 6:24; Mark 11:22; Luke 2:14).
The New Testament especially emphasizes God's love for the world and his desire to save all people (John 3:16; Rom 5:5,5:8; Phiippians 4:19, 1 John 4:7-9). God also expects behavior from people that accords with his righteous standards (John 6:29; Acts 8:21, 24:16; 2 Cor. 9:7; 1 Thess. 4:9; Jas. 1:27; 1 John 3:9) and will judge wrongdoers (Rom. 2:16, 3:19). The New Testament emphasizes that God chose to reveal himself to humanity by means of the second person of the Trinity, the Son, taking on human flesh in the person of Jesus Christ (e.g. John 1:1-3, 14).
These verses are among those used to support this doctrine:
- John 3:35 - "The Father loves the Son and has placed everything in his hands." (John the Baptist)
- John 7:16: "My teaching is not my own. It comes from him who sent me."
- John 14:9-10: "Anyone who has seen me has seen the Father. How can you say, 'Show us the Father?' Don't you believe that I am in the Father, and that the Father is in me?"
- Romans 1:17: "For in the gospel a righteousness from God is revealed."
God in Early Christian Theology
The early Greek fathers made extensive use of reason and philosophy in their attempt to defend Christianity in the cultured world around them. Justin Martyr emphasized the ineffability, omnipotence and impassibility of God, while Athenagoras and Theophilus focused on God's simplicity, indivisibility and universal providence. Irenaeus developed his doctrine of God in reaction against the Gnostics, and thus emphasized the self-sufficiency and perfection of the one God. By the time of the Council of Nicea, that the bible taught that the chief divine attributes of God as eternity, immutability, omniscience and omnipotence were undisputed by all Christians.
A Personal God
The Christian God is a personal God. This does not mean that God is a human being, but that God has "personality" and is capable of relationships with other personal beings. This is seen clearly in both the Old and New Testaments, in which God is described in strongly personal terms (e.g. father, shepherd and establishes personal relationships with human beings.
Is the Christian God Masculine?
Throughout the Christian Bible, masculine language is used to refer to God. The Greek word for God (theos), the pronouns used to refer to God, and most of the analogies used to describe God are masculine. But it has never been a part of Christian doctrine that God is male, or that God has gender at all. In Genesis 1:27, God creates both male and female in his image. God thus incorporates the fullness of both masculinity and femininity within himself. In fact, the notion of God having gender is a pagan one, associated especially with the fertility cults that were explicitly rejected by the authors of the Old Testament.
So why is masculine imagery used for God? Christians explain that traditionally masculine human roles seemed to the biblical writers to provide the best analogies for God. They used masculine language because they wished to emphasize that God is a leader, a provider, and a disciplinarian, all of which were roles associated with men. But as Oxford professor Alister McGrath points out:The statement that "a father in ancient Israelite society is a suitable model for God" is not equivalent to saying that "God is male" or that "God is confined to the cultural parameters of ancient Israel."
In light of increasing concerns about language and gender, some Christian denominations have made official statements rejecting the notion that God is male (or female). One notable example is the 1994 Catechism of the Catholic Church, which explains:By calling God "Father," the language of faith indicates two main things: that God is the first origin of everything and transcendent authority; and that he is at the same time goodness and loving care for all his children. God's parental tenderness can also be expressed by the image of motherhood, which emphasizes God's immanence, the intimacy between Creator and creature. ... We ought therefore to recall that God transcends the human distinction between the sexes. He is neither man nor woman; he is God. He also transcends human fatherhood and motherhood, although he is their origin and standard. 
Many individual Christians have also begun to emphasize the importance of gender-inclusive language when talking about God. This usually involves replacement of masculine terms such as "Father" and "Son" with neutral terms such as "Parent" and "Child," and avoidance of pronouns for God altogether, both of which can present a linguistic challenge. It is also controversial - critics argue that these changes can result in loss of meaning and do not give enough weight to God's self-revelation. 
Denominational Statements about God
Although the theological beliefs described later in this article are broad enough to apply to all mainline Christian denominations, it may be of interest to the reader to explore the manner in which various confessional agencies have chosen to officially express these beliefs. Following, therefore, are excerpts from official doctrinal statements about God made by several Christian traditions.
United Methodist Church:With Christians of other communions we confess belief in the triune God - Father, Son, and Holy Spirit. This confession embraces the biblical witness to God's activity in creation, encompasses God's gracious self-involvement in the dramas of history, and anticipates the consummation of God's reign.
Assemblies of God:The one true God has revealed Himself as the eternally self-existent "I AM," the Creator of heaven and earth and the Redeemer of mankind. He has further revealed Himself as embodying the principles of relationship and association as Father, Son and Holy Ghost.
Lutheran Church - Missouri Synod:On the basis of the Holy Scriptures we teach the sublime article of the Holy Trinity; that is, we teach that the one true God, Deut. 6:4; 1 Cor. 8:4, is the Father and the Son and the Holy Ghost, three distinct persons, but of one and the same divine essence, equal in power, equal in eternity, equal in majesty, because each person possesses the one divine essence entire, Col. 2:9, Matt. 28:19.
Catechism of the Catholic Church:The Church never ceases to proclaim her faith in one only God: Father, Son and Holy Spirit.
Southern Baptist Convention:There is one and only one living and true God. He is an intelligent, spiritual, and personal Being, the Creator, Redeemer, Preserver, and Ruler of the universe. God is infinite in holiness and all other perfections. God is all powerful and all knowing; and His perfect knowledge extends to all things, past, present, and future, including the future decisions of His free creatures. ... The eternal triune God reveals Himself to us as Father, Son, and Holy Spirit, with distinct personal attributes, but without division of nature, essence, or being.
Presbyterian Church (USA):Through the grace of our Lord Jesus Christ, the love of God, and the communion of the Holy Spirit, we trust in the one triune God, the Holy One of Israel, whom alone we worship and serve.
Greek Orthodox Archidiocese of America:While the inner Being of God always remains unknown and unapproachable, God has manifested Himself to us; and the Church has experienced Him as Father, Son, and Holy Spirit. The Doctrine of the Holy Trinity, which is central to the Orthodox Faith, is not a result of pious speculation, but the over whelming experience of God. The doctrine affirms that there is only One God in whom there are three distinct Persons. In other words, when we encounter either the Father, the Son, or the Holy Spirit, we are truly experiencing contact with God. References and Sources
- McGrath, Christian Theology, 240.
- Revelations of Divine Love (1373), quoted in McGrath, 242.
- Quoted in McGrath, 241.
- See e.g., John Cooper, "Gender-Inclusive Language for God," Theological Forum, Vol. 26, No. 3 & 4, December 1998.
- The Nature and Attributes of God - Catholic Encyclopedia
- Attributes of God - Christianity Today Library
- Questions about God - GotQuestions.org
- Sermons on the Character of God - PreachingTodaySermons.org
- Why God is Father, not Mother? - Sermon outline from PreachingTodaySermons.org.
- Why the Christian God is Impossible - Atheist Soapbox
- Christian Theology: An Introduction, Third Edition by Alister McGrath.
- A History of Christian Theology by William C. Placher.
- Readings in the History of Christian Theology.
- Charts of Christian Theology and Doctrine.
- A History of Christian Thought by Justo Gonzalez.
- The Question of God: C.S. Lewis and Sigmund Freud by Armand Nicholi.
- The Christian Tradition: A History of the Development of Doctrine (100-600 AD) by Jaroslav Pelikan.