The Gospel in the Old Testament
Rom 1:2-4
the gospel he promised beforehand through his prophets in the Holy Scriptures regarding his Son, who as to his human nature was a descendant of David, and who through the Spirit of holiness was declared with power to be the Son of God by his resurrection from the dead: Jesus Christ our Lord.
There are many prophecies of the Messiah, such as Isaiah chapter 53, Psalm 22 and Daniel 9:25,26. But the first was Genesis 3:15 prophesying of one who would be born of a woman and who would destroy the power of the devil.
Concerning his human nature, Jesus was a descendant of David. While the genealogy of his legal guardian, Joseph, is mentioned in Matthew chapter 1, his physical genealogy from Mary in Luke chapter 3 goes all the way back to Adam. Jesus Christ came in the flesh, having the same human nature that we all share with its flaws. In fact the word for "human nature" here is the Greek word "Sarx", of which out of the 23 times it's used in Romans the NIV translates "sinful nature" 13 times. Thus the saying in
Hebrews 4:15
"We do not have a high priest who is unable to sympathize with our weaknesses, but we have one who has been tempted in every way, just as we are-- yet was without sin." For the temptations we suffer as humans are primarily due to our sinful human nature, of which Jesus also shared. Though there are those who deny Christ's human nature,
"every spirit that does not confess that Jesus Christ has come in the flesh (sarx) is not of God."
1John 4:3a
Jesus' resurrection from the dead was his primary credential - the forensic evidence validating his testimony. This fact is mentioned every time the gospel is preached in the Bible.
"He has given proof of this to all men by raising him from the dead."
Acts 17:31