What is the gospel really, beyond just believing facts about Jesus?

A lot of people today believe in the gospel like it's a set of theological propositions: Christ is the incarnate God who died on the cross for their sins, resurrected on the third day, ascended into heaven. But there is a whole other meaning to the gospel, and that is actually to become the gospel, to become like Christ. This is what the Holy Fathers describe as theosis, and I would say it is the only valid form of Christianity.

Years ago somebody said to me, Christ died on the cross so that we don't have to. I remember thinking, the apostles definitely did not get that memo, because eleven of the twelve were martyred. Do you actually believe that you have to become like Christ in order to be saved, that salvation is the process of becoming like Christ? It is not some whimsical theological assent, as if you die and get a multiple-choice test and just have to answer the theological questions correctly.