The gospel preached to Adam and Eve, about a coming savior that would put to death the one who brought the deception and the source of hostility between God and man.
This savior would defeat the serpent and his offspring (the wicked demons and those who follow after them). And he ultimately would restore what they had lost, access to the source of life (fellowship with God).
Luke’s geneology points us back to this garden scene, by saying Christ is the son of Adam (i.e., the promised seed).
Notice how Luke goes back to the OT, whereas Matthew starts in the OT. The two genealogies point in different directions.
This savior is also God the Son, as the NT clearly teaches.
Jesus preached about His death and resurrection, before it happened
First to the Jews, since they had the Scriptures which gave the revelation. Then Christ came to remind them and say He was the fulfillment of those promises to save a people