The map of the cosmos according to Christianity has either three or four territories, depending on the version of Christianity. All Christianities identify heaven, the earth, and hell. Catholicism also has a place called purgatory, an idea that was rejected by Protestantism during the Reformation. (The Orthodox Church has a similar idea, but it is not as highly developed.) This depiction of the cosmos is only temporary. At some point in the future, God will bring about a Great Day of Judgement, where all humans (and some angels) will be judged. At that point the Kingdom of God will be ushered in and the earth will be swept away. So like Islam, Christianity believes in a cosmos with one form now, but which is moving towards a future in which it will be transformed.
Heaven is the dwelling place of God and his angels. God is conceived as a single being who has three parts. This tri-partite nature is called The Trinity. (For further discussion, see below.) God is the creator of all things and he himself (the gender designation is only convention) was not created. Angels are spiritual beings who were created by God to be his messengers and helpers.
Earth is the place of creation; every being there was created by God. The primary species of creation is human beings. They are the only creatures (remember, "creature" means "created being") who have free will and thus can choose to follow God's will (i.e., do good) or to do evil. Indeed, according to Christian belief, the ability of humans to choose good or evil requires that God judge the actions of their one life (Christianity has no reincarnation as in Hinduism) after their death. This will be done at the great Judgement Day in the future. Thus, after the death of human beings, they wait until Judgement Day for to discover whether they will be rewarded with eternal life in heaven, or punished with eternal torture in hell.
The question of what humans do between their death and the Judgement Day has been given many answers by Christian theologians. One answer is that they simply "sleep" until they are resurrected for judgement. Another answer is that they go to purgatory; this idea is important in Catholicism. Purgatory is a place for the souls of people who lived a "good life," but who still had some sin they had not purged upon death. The fire of purgatory thus purifies them (see 1 Corinthians 3:11-15) so that at Judgement Day they may go to heaven.
There are two classes of Christians who go straight to heaven. The first are the martyrs, those who were killed for their faith in Christ. The second are the saints. Saints are people who lived an exemplary, god-like life, being blessed by a nearness to God. Indeed, they are seen in some ways as a reflection of Christ on earth--continually living a virtuous life dedicated to God. Upon death, saints ascend directly to heaven. There, according to the Western Church, they function as helpers of human beings and as beings who intercede with God on behalf of individuals and groups. According to the Eastern Church, they function more importantly as examples of how to live a holy life. (For more information about saints in general and in the Orthodox church, go here. For more information about Catholic beliefs about saints, go here.)
Hell, the third of the cosmic territories, is populated primarily by fallen angels (see below) and by demons. In the future, after the Judgement Day, those humans who will spend eternity in punishment will also be there.
Christianity believes that there are actually two stages to the Kingdom of God that will come. At first, the Kingdom of God will be established on the earth, and only later followed by a Kingdom of God in heaven. The earthly kingdom will come about, according to the Letters of Paul and the Revelation of John, when the messiah (Jesus Christ) returns to the earth (a.k.a. the Second Coming). The Christians will then be raised from the dead and will rule with Christ for an extended period of time (1000 years according to Revelation). After that, all humanity will be resurrected and judged at the Great Judgement Day, the righteous will live eternally with God and the wicked will receive eternal punishment in hell. At this point, a new heaven and a new earth will be created and the heavenly Jerusalem will descend to the earth.
In Christianity, God exists in three parts: the Father, the Son, and the Holy Ghost. These are three aspects of a single, unitary being; they are not three separate beings. Thus Christianity is form of monotheism. To address the paradox of a three-in-one being, Christianity in the first few centuries developed the doctrine of the Trinity. To explain the Trinity, the three aspects of God must first be described.
God the Father can in many ways be identified as the God of the Old Testament. He is the creator of the cosmos, the Lord of History, and the divine Judge. His "personality" is essentially that found in the Old Testament. It is he who acted with Israel, choosing them as his people, give them his revelation, and so on.
God the Son is Jesus, who is seen as the messiah, i.e., as the "Christ." His death as a sacrifice on the cross and his subsequent resurrection demonstrates God's triumph over sin and death, and thus provides salvation for all humanity. God the Son's birth as a human being brought about a combination of the human and divine nature into one being. The definition of the exact nature of this phenomenon caused great controversy in the early church until it was decided at the Council of Chalcedon in 451. On the one hand, the delegation from Antioch (Syria) held that humanity of Christ was most important, and they proposed that Jesus should essentially be understood as two essences in two persons, the human essence never mingling with the divine essence. Taking their lead from Mark's description of Jesus' baptism, they believed that Jesus had been a human who at baptism had a divine character imposed upon him (Mark 1:9-11). On the other hand, the delegation from Alexandria (Egypt) argued that Jesus' divinity was most important, and that his humanity was almost secondary. Taking their queue from John 1:1-4, in which Jesus is described as the pre-existing Logos who is identical to God, the Alexandrians believed that Jesus had one nature and that it was divine. The Council of Chalcedon found a compromise and defined Jesus as having two essences (one human and one Divine) which were intermingled in one person. (To read the decision of Chalceon, click here.) Although accepted by most Christians, the Christians of the South-east rejected this definition and split from the main body of the church. These are known as the monophysite churches ("one essence") and include the Coptic, Armenian, and the Ethiopic Churches.
God the Holy Spirit represents the continuing activity of God within the Church. It is the strength of the Spirit that imbues each Christian with new life and leads to the coming Kingdom. On the one hand, the Spirit works to uphold law, tradition, custom and authority. On the other hand, it sometimes seeks to create change by breaking law, tradition, customs or authority. It grants prophecy which brings new revelation and it forms the charismatic power transferred from bishop to bishop through the laying on of hands. In these two ways, it continues God's activity in the world.
The Trinity is the joining of these three "things" (what are they? beings? persons? aspects? etc.) into one. And of course having just "One" was important to early Christianity, for monotheism is what distinguished them from paganism. But how to define three-in-one? The conflict was started by an Alexandrian theologian named Arius. He held that God the Father was the one true God--no questions about monotheism here. Jesus was a secondary god, the first of the Father's creations. The main problem with this formulation was that it was "Jesus the creation" who died on the cross rather than "Jesus the God." Where then was the power to bring salvation? The proposal from Athanasius won the day. He argued that that God was one substance and three persons. This formulation preserved both monotheism and the three equal beings of the Godhead. Arianism was condemned by the Council of Nicea as heresy. This definition began to become unraveled--particularly through court politics--until it was finally reaffirmed at the Council of Constantinople in 381. (Read the Nicene Creed, the Constantinople version reaffirmed in 381, and a version of the statement by Athanasius. For an Orthodox explanation of the Trinity, go here.)
A question about the nature of evil often heard is: if God is good, then how can he create evil? The answer is that he did not create evil; evil is the result of free will. Traditionally, Christianity has seen evil in two places, namely, within each individual human being, and outside each human. Each is a result of the exercise of free will.
Each person's tendency to do evil--i.e., to do something against God's will--stems from an idea Christianity has traditionally called Original Sin. The concept of Original Sin is explained as follows: the prototypical human beings, Adam and Eve, used their free will to rebel against God by disobeying his commands. Since they were the "founders of humanity," this act imbued all humanity with a tendency to rebel against God. This tendency is called Original Sin and means that all human beings are by default evil and assumes that without outside influences, they will more readily commit sin than do good. This notion that humans are naturally born evil as a result of the free will of the archetypal human couple is part of Christianity's understanding of the human problem. For further discussion, see below.
Another being who exercised free will was an angel by the name of Satan. He was one of God's most ardant worshippers and was widely known among the angelic community for his beauty, his voice, and his ability to worship God. Numbers of angels would gather around him just to study his example for worshipping God. This attention filled him with pride and he rebelled against God. God punished him by kicking him and his followers out of heaven. Although they reside in hell, they are not yet imprisoned there. In fact, Satan is known as the Lord of the Earth. He and his followers use their position to tempt human beings to follow him and to rebel against God's wishes, that is, to do evil.
Christianity's definition of the Human Problem becomes clear from the above description of the cosmos. Humankind is inherently sinful and every individual is destined for a Judgement that will condemn them to eternal punishment in hell. This is because humans are incapable of overcoming their sinful nature, try as they might, without divine help. To make matters worse, Satan and his allies continually work to deceive Christians and lead them away from God.
The solution the human problem put forward by Christianity has two parts, both of which are carried out by Jesus, i.e., God the Son. First, every individual can be rescued from their sinful nature by Jesus' sacrifice on the cross. Second, the rule of Satan will be overcome by Christ's second coming and his establishment of the Kingdom of God on earth.
The death of Jesus Christ, God the Son, on the cross and his following resurrection provides the means of salvation potentially available for all people. If they will simply accept the results of that act (the manner of "accepting" varies among the different forms of Christianity), they will be saved, reckoned as righteous, and will join God in eternal life after the Judgement Day. Jesus' death and resurrection can be understood in several different ways. First, it is a triumph over Satan and shows that he cannot overcome God's power. Thus, by joining themselves to God, believers can overcome Satan's power. Second, just a the first Adam brought about humankind's sinful nature by his Original Sin, Jesus is the second Adam who brought a means of atoning for that sin. Since no individual could overcome the result of Adam's deed, Jesus' sacrifice brings atonement that is available to everyone. Third, Jesus' sacrifice is thought to have paid the penalty for everyone's sins.
Jesus' coming in the future will usher in the Kingdom of God on earth. According to the Book of Revelation, this Kingdom will last 1000 years during which Satan will be bound. All Christians will be resurrected and will share in the glories of that kingdom. After the 1000 years, there will be a final battle, Satan will be overcome and permanently caste into hell. There will be a Day of Judgement and all the wicked will be punished with eternal life in hell. Those judged as righteous (the Christians) will be given eternal life with God in the New Heaven and New Earth.