Because of Adam's disobedience and the punishment of mortality, humanity needs redemption. Jesus, the "Second Adam," restores men to eternal life.
Was it fair to punish all humans for Adam's sin? It is not exactly a punishment, because immortality was never an entitlement to begin with. The Law of Works (also called the Law of Reason or Nature) is the only rational way to live. It applies to Gentiles as well as Jews because it can be discovered by reason and natural conscience. If people could obey it perfectly, they would be immortal, but they cannot. God cannot soften the law, because it would be against his nature and a slippery slope towards immorality.
Faith makes up for shortcomings of works. Under the Law of Faith, a Christian is only required to believe that Jesus was the Messiah. (The Epistles include additional doctrines but they are not central to the faith.)
The Messiah is the Deliverer and a metaphorical King. Jesus himself did not claim publicly to be the Messiah to avoid attracting negative attention from Jews who might have killed him. Until the Last Supper, Jesus did not make the claim to his own Apostles. Instead, Jesus performed miracles so people would realize he was the Messiah.
Jesus chose Apostles who would trust him rather than question him and who would preach the uncomplicated doctrine that he was the Messiah. He chose, as Locke put it, "a company of Poor, Ignorant, Illiterate Men--but meer Children."
Acknowledging a popular claim that the identification of Jesus as the Messiah is merely a "historical faith," not a "saving faith," Locke insists the Bible says it is sufficient and says he is not aware of any other doctrine that would be a "saving faith."
God wants people to come heaven so they can praise him. He accepts people as long as they profess allegiance to Jesus the Messiah-King and make a sincere effort to follow to the Law of Works (otherwise, the Law of Faith would be an excuse for anarchy). Jesus required good works in the Sermon on the Mount and demanded repentance for sins. God would not demand more than is possible for people who lived before Jesus existed.
People have always wanted to know their moral duty. Philosophers laid codes, but none had authority, and if people cherrypicked their moral beliefs from different philosophers, they would be accountable to no one. No one before Jesus managed to present the Natural Law in its entirety. "He was sent by God: His Miracles shew it; And the Authority of God in his Precepts cannot be questioned," Locke wrote. Another reason no other culture managed to develop virtue was that none had laid a firm belief in eternal life. It's not enough to say virtue is its own reward; the only way to encourage people to be virtuous is to impress upon them a belief in Heaven and Hell.
John Locke. The Reasonableness of Christianity, as Delivered in the Scriptures. (1695) Ed. John C. Higgins-Biddle. Oxford: Clarendon Press, 1999.
This summary was written in 2005, along with a series of other 500-word summaries of philosophy books, as an exercise in brevity.