The main issue that Christianity deals with is "sin".
"Sin" is breaking God's law and God's law can be summarized in the words "care for other people as you care for yourself".
Jesus summed up His teaching with the words "All things whatsoever you would that people should do to you, do the same to them: for this is the law and the prophets".
Since none of us are able to do this as perfectly as God requires us to do it, the main message that Jesus and His disciples preached is "repent".
"Repent" means to be truly sorry about sinning and to make a firm decision that, with God's help, you are going to stop.
If a person truly repents, they will receive faith. This will make them believe in Jesus Christ without any doubt. And it is this faith that gives a person everlasting life. But a person can not receive this faith unless they first of all repent.
Jesus said "For God so loved the world, that He gave His only begotten Son, that whoever believes in Him should not perish but have everlasting life.
For God did not send His Son into the world to condemn the world, but that the world through Him might be saved.
Whoever believes in Him is not condemned; but whoever does not believe is condemned already, because they have not believed in the name of the only begotten Son of God.
And this is the condemnation, that the light has come into the world, and people loved darkness rather than light, because their deeds were evil.
For everyone practicing evil hates the light and does not come to the light, lest their deeds should be exposed.
But whoever does the truth comes to the light, that their deeds may be clearly seen, that they have been done in God".
Scripture references are Matthew 4:17, Matthew 7:12, Acts 3:19, Acts 26:20, John 3:16-21.