The 12 were not preaching Jesus's death burial and resurrection during the 4 gospels.
They were preaching the gospel of the kingdom, the content can be summarized in Matthew 10:7.
That will be the very same gospel of the kingdom that will be preached to Israel during the Tribulation (Matthew 24:14).
Israel will need to acknowledge Jesus as their Messiah and Son of God for the end to come, not that he died for their sins and rose again for their justification, even though the latter is still true.
The Old Testament Scriptures predicted the Messiah from the Garden! The LORD said in Genesis 3:15:
“I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel.” The bruising of Satan’s head by Christ broke the power of sin over the penitent sinner. Jesus made a way of escape for us. He didn’t leave Adam and Eve – our first parents – naked, hopeless and condemned. No! But by a gracious act of mercy He created a way of escape.
Genesis 3:21 reveals,
“Unto Adam also and to his wife did the LORD God make coats of skins, and clothed them.” The Lord removed their old tattered garments and gave them a divine covering. Here is the first blood sacrifice. Significantly it was made by Almighty God. This renewed them and brought them back into communion with God. They were new creatures. The animal skins did not just cover their physical bodies but also represented the sacrifice of the innocent animal’s blood as a means of covering sin. We know that their progeny were familiar with this procedure of animal blood sacrifice (Genesis 4:4-5), for we see it also in the story of righteous Abel.
Noah later followed suit (Genesis 8:20-21). God commanded Abram to prepare animals to sacrifice. He then performed the sacrifice, cutting the animals into two pieces (Genesis 15:9-10). A sacrifice system was formally instituted under Moses that involved official offices, detailed furniture, elaborate procedures, and particular animals (the book of Leviticus). All these sacrifices pointed to the coming Lamb of God. This bloody scarlet thread ran throughout the Old Testament and ends up at the cross of Christ.
Paul, in Galatians 3:8-9, shows that Abraham was saved through the same inspired Gospel as we are today. It was the good news of salvation, by grace, through faith in Christ. It says:
“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. So then they which be of faith are blessed with faithful Abraham.”
The same glorious Gospel runs from the Old to the New Testament. It involves one ongoing progressive harmonious revelation of the person and work of Christ. He is man’s only redeemer. He is man’s only hope. Notwithstanding, the Old Testament saints were looking at our Savior from a more obscure perspective, and their revelation of Him was more veiled than that of the new covenant saints. However, they embraced the same overriding life-changing message.
The old covenant saints were looking forward with anticipation to Christ’s earthly assignment on man’s behalf, while the new covenant Christian is looking back to His victorious earthly ministry. Salvation came in both testaments through the enlightening power of the Gospel. We see in this passage that it the glad tiding of good news that we enjoy today was proclaimed to Abraham and he embraced it by faith.
Adam Clarke expounds this passage: “As God intended to justify the heathen through faith, he preached the Gospel that contains the grand display of the doctrine of salvation by faith, before, to Abraham, while he was in his heathen state; and thus he is called the father of believers: therefore it must refer to them who shall believe the same Gospel among the Gentiles; and, as the door of faith was open to all the Gentiles, consequently the promise was fulfilled: In thee shall all the nations of the earth be blessed.”
Repeated Scripture shows the continuity between the old covenant Gospel message and the new covenant Gospel message. It also shows the harmony between the people of God in the Old Testament and the people of God in the New Testament.
1 Peter 4:4-6,
“they think it strange that ye run not with them to the same excess of riot, speaking evil of you: Who shall give account to him that is ready to judge the quick and the dead. For for this cause was the gospel preached also to them that are dead, that they might be judged according to men in the flesh, but live according to God in the spirit.”
Previous generations have heard the good news of the Lord Jesus Christ. Those that had eye to see embraced Him, those that were blind rejected Him. There is one Gospel that has gradually developed through clearer revelation. All the truth we find in the New Testament can normally be found in the Old Testament, albeit sometimes veiled. This not does in any way suggest 2 Gospels. There is only one Gospel, one faith, one overall covenant of grace, one election and one salvation.
Hebrews 3:17-19 4:1-2:
“with whom was he grieved forty years? was it not with them that had sinned, whose carcases fell in the wilderness? And to whom sware he that they should not enter into his rest, but to them that believed not? So we see that they could not enter in because of unbelief. Let us therefore fear, lest, a promise being left us of entering into his rest, any of you should seem to come short of it. 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.”
The writer to the Hebrews is here showing us that the Gospel was indeed preached onto natural Israel in the Old Testament. This is the same Gospel that we have today. This Gospel produced the same fruit then, in those that got it, as it does in us today. Those who rejected that liberating message back then faced the same awful consequences as those who do the same today. The Gospel in essentially the good news of Jesus Christ. He is man’s only Savior. He is man’s only hope. The Old Testament saints looked forward by faith to His appearing. We today look back and rejoice in His appearing 2000 years ago.
The Old Testament sets forth a Gospel theme consistent with the New Testament that men were saved from sin by grace through saving faith in the Lord and His promises.
The Gospel message was a prophetic message predicting the sufferings of the Messiah and the glories that would follow. Christ was the central focus of the Old Testament. His life, death and resurrection are man’s only hope – past, present and future.