Christianity and Christians - What do they believe?

Christians have been getting wide coverage in the media recently following the militant rampage through northern Iraq and in Syria. Islamist extremists have conducted various atrocities against minority groups, including Christian communities. For many non-Christians, there are questions regarding the group. Who are they? What do they believe? What does it mean to be a Christian? 

There are many denominations but the majority of Christians adhere to the Gospel and these general principles.

Mankind has rebelled against God and adopted the ways of the world:

As for you, you were dead in your transgressions and sins, 2 in which you used to live when you followed the ways of this world and of the ruler of the kingdom of the air, the spirit who is now at work in those who are disobedient. 3 All of us also lived among them at one time, gratifying the cravings of our flesh[a] and following its desires and thoughts. Like the rest, we were by nature deserving of wrath. – Ephesians Chapter 2

Rescue, Resurrection and Response – Christians believe God sent His Son, Jesus, to take on our sins and through him receive salvation. Jesus lived, this is documented. Christians believe He is God as well. Tricky connection but think of them as connected through the Spirit. So how does one become a Christian? Its simple, just repent of your sins (say you're sorry) and ask Jesus to be your saviour. No other actions required. If you pray these words and believe them, welcome to the Christian family.

4 But because of his great love for us, God, who is rich in mercy, 5 made us alive with Christ even when we were dead in transgressions—it is by grace you have been saved. 6 And God raised us up with Christ and seated us with him in the heavenly realms in Christ Jesus, 7 in order that in the coming ages he might show the incomparable riches of his grace, expressed in his kindness to us in Christ Jesus. 8 For it is by grace you have been saved, through faith—and this is not from yourselves, it is the gift of God— 9 not by works, so that no one can boast. 10 For we are God's handiwork, created in Christ Jesus to do good works, which God prepared in advance for us to do. – Ephesians Chapter 2