There is more than one gospel in the Bible. It is imperative to comprehend not only to whom each gospel was given, but how they differ. It is also of the upmost importance to know which gospel is for our salvation unto eternal life.
Galatians 3:8 And the scripture, foreseeing that God would justify the heathen through faith, preached before the gospel unto Abraham, saying, In thee shall all nations be blessed.
The gospel was given to Abraham. A gospel is glad tidings, good news. The obligation of mankind is to believe what God said. Abraham believed that God would bless all the nations through him.
Genesis 15:5 And he brought him forth abroad, and said, Look now toward heaven, and tell the stars, if thou be able to number them: and he said unto him, So shall thy seed be.15:6 And he believed in the LORD; and he counted it to him for righteousness.
It is always and only by faith that God saves, by believing God. It is always and only by grace that God saves in doing for us what we are incapable of in and of ourselves.
The gospel was given to Israel in the wilderness.
Hebrews 4:1Let us therefore fear, lest, a promise being left us of entering into his rest, any of you should seem to come short of it.
4:2 For unto us was the gospel preached, as well as unto them: but the word preached did not profit them, not being mixed with faith in them that heard it.
What gospel was the writer of Hebrews referring to?
Deuteronomy 1:8 Behold, I have set the land before you: go in and possess the land which the LORD sware unto your fathers, Abraham, Isaac, and Jacob, to give unto them and to their seed after them.
The gospel of entering into God's rest through belief in Hebrews 4 is not the same gospel given to Israel in the wilderness to possess the land. It can be seen from this comparison alone that there is more than just one gospel.
Jesus preached the gospel of the kingdom.
Mark 1:15 And saying, The time is fulfilled, and the kingdom of God is at hand: repent ye, and believe the gospel.
Matthew 4:17 From that time Jesus began to preach, and to say, Repent: for the kingdom of heaven is at hand.
The Davidic kingdom was promised to Israel, wherein, they would inherit an earthly kingdom and reign with Christ (Daniel 2:44 Revelation 20:4). Israel was to believe on the name of Jesus, that Jesus was the Son of God, the Messiah. Placing faith and trust in Jesus and his name would justify them unto eternal life (John 20:31). Obeying his commandments would physically save them to enter the kingdom (Matthew 5:20). Upon belief the kingdom would have been ushered in.
The Jews had to only believe on the name of Jesus to have eternal life.
[John 20:31 KJV] 31 But these are written, that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name.
[John 3:16, 18 KJV] 16 For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life. ... 18 He that believeth on him is not condemned: but he that believeth not is condemned already, because he hath not believed in the name of the only begotten Son of God.