Mankind had offended God to the point of needing to be rescued from eternal punishment for their sins. It all started with Adam and Eve our first parents, who first offended God but who received the promise of Salvation.
In the book of Genesis [Genesis 3:15], the proto evangelium tells us that God threatened the serpent [the devil] to be crushed by the Woman [The Blessed Virgin] whose heel would be bruised [Our Lord Jesus by his death].
God foretold the enmity between the Woman and her offspring
that is Mary, her son Jesus and all her new children [the children of God] and the serpent
and his offspring, that is the devil and all those who reject God and become his children.
In simple words God promised to send a woman whose son would be Our Saviour because by his death he would overcome the devil. [Genesis 3:15]
The Lord said to Moses, I will raise a prophet like you , one from your own people. He will speak in my name all that I command Him, I will put my own words in his mouth. If anyone does not listen to Him, that person will be cut off from his people. [Deuteronomy 18:15-19]
Other prophets received the message of the coming of the Messiah, He would be the one to liberate the people of God from slavery to other nations.
Through the prophet Micah, it was revealed that Jesus would be born in Bethlehem, and that He would be a powerful king and also a shepherd, He would bring peace to the nations. [Micah 5:2-5]
Therefore the Lord himself shall give you a sign. 1 Behold a virgin shall conceive, and bear a son, and his name shall be called Emmanuel [Isaiah 7:14]
For a CHILD IS BORN to us, and a son is given to us, and the government is upon his shoulder: and his name shall be called, Wonderful, Counsellor, God the Mighty, the Father of the world to come, the Prince of Peace. [Isaiah 9:6]