0 What is the Gospel?

The gospel is the singularly most important communication of God to man.  In Jesus, who is God the Son, we have the revelation of God’s love and sacrifice that saves us from God's righteous judgment upon sinners.
If you are not a Christian and want to know how to be forgiven of your sins and follow Christ, or are just curious to know what the Christian gospel is, then this is for you.
The Bible tells us what the gospel is in 1 Cor. 15:1-4,

0 Essential Doctrines of Christianity

The Bible itself reveals those doctrines that are essential to the Christian faith.  They are 1) the Deity of Christ, 2) Salvation by Grace, and 3) Resurrection of Christ, 4) the gospel, and 5) monotheism.  These are the doctrines the Bible says are necessary.  Though there are many other important doctrines, these five are the ones that are declared by Scripture to be essential.  A non-regenerate person (i.e., Mormon or Jehovah's Witness, atheist, Muslim), will deny one or more of these essential doctrines.   Please note that there are other derivative doctrines of scripture that become necessary also, the Trinity being one.