The Apostle Paul gives a wonderful summation of the Gospel in 1 Corinthians 15 where he states that it is that, "Christ died for our sins in accordance with the Scriptures, that he was buried, that he was raised on the third day in accordance with the Scriptures, and that he appeared to Cephas, then to the twelve. Then he appeared to more than five hundred...And if Christ has not been raised, then our preaching is in vain and...your faith is futile and you are still in your sins...If in this life only we have hoped in Christ, we are of all people most to be pitied. But in fact Christ has been raised from the dead, the firstfruits of those who have fallen asleep. For as by a man came death, by a man has come also the resurrection of the dead. For as in Adam all die, so also in Christ shall all be made alive."
Put simply, as Paul writes in Ephesians 2:8-9, "for by grace you have been saved through faith. And this is not your own doing; it is the gift of God, not a result of works, so that no one may boast."