The Nugget:
Mar 1:15 And saying, The time is fulfilled, and the kingdom of God is at hand: repent ye, and believe the gospel.
The Question:
Which “gospel” was Jesus referring to in Mark 1:15 when none of the New Testament texts had yet been written?
My Answer:
PixelMistakePicasso You are asking the wrong question. This is probably the case because you have not taken the time and effort to read the New Testament and the Bible as a whole enough to understand its central themes.
The Kingdom of God is a central theme across all of the Bible, Old Testament and New. Very few Christians, even Bible believing Christians, have had or taken the opportunity to study this subject thoroughly. There is a lot more to it than you likely know at the present time.
There is a Bible scholar from the nineteenth century by the name of George N. H. Peters who wrote three whole volumes (each about 700 pages long) carefully exploring and explaining the subject of the Kingdom of God. His volumes are titled The Theocratic Kingdom of our Lord Jesus Christ.
In a (long!) sentence summary I can explain that Peters shows that the Kingdom was promised in the Old Testament and described in the Abrahamic Covenant and the Davidic Covenant as to its provisions, which are further amplified in the Prophets, then offered and preached by our Lord Jesus Christ (as in Mark 1:14, 15) and His disciples and Apostles, then rejected by the Jewish leadership, then postponed (Matthew 21:43, a point most students of Scripture, even Bible scholars, have missed), then promised to take place on this earth upon the requisite repentance of the nation of Israel as carefully explained by Peter in Acts 3:13-26,
Act 3:13 The God of Abraham, and of Isaac, and of Jacob, the God of our fathers, hath glorified his Son Jesus; whom ye delivered up, and denied him in the presence of Pilate, when he was determined to let him go.
Act 3:14 But ye denied the Holy One and the Just, and desired a murderer to be granted unto you;
Act 3:15 And killed the Prince of life, whom God hath raised from the dead; whereof we are witnesses.
Act 3:16 And his name through faith in his name hath made this man strong, whom ye see and know: yea, the faith which is by him hath given him this perfect soundness in the presence of you all.
Act 3:17 And now, brethren, I wot that through ignorance ye did it, as did also your rulers.
Act 3:18 But those things, which God before had shewed by the mouth of all his prophets, that Christ should suffer, he hath so fulfilled.
Act 3:19 Repent ye therefore, and be converted, that your sins may be blotted out, when the times of refreshing shall come from the presence of the Lord;
Act 3:20 And he shall send Jesus Christ, which before was preached unto you:
Act 3:21 Whom the heaven must receive until the times of restitution of all things, which God hath spoken by the mouth of all his holy prophets since the world began.
Act 3:22 For Moses truly said unto the fathers, A prophet shall the Lord your God raise up unto you of your brethren, like unto me; him shall ye hear in all things whatsoever he shall say unto you.
Act 3:23 And it shall come to pass, that every soul, which will not hear that prophet, shall be destroyed from among the people.
Act 3:24 Yea, and all the prophets from Samuel and those that follow after, as many as have spoken, have likewise foretold of these days.
Act 3:25 Ye are the children of the prophets, and of the covenant which God made with our fathers, saying unto Abraham, And in thy seed shall all the kindreds of the earth be blessed.
Act 3:26 Unto you first God, having raised up his Son Jesus, sent him to bless you, in turning away every one of you from his iniquities.
Notice particularly Acts 3:19-21,
Act 3:19 Repent ye therefore, and be converted, that your sins may be blotted out, when the times of refreshing shall come from the presence of the Lord;
Act 3:20 And he shall send Jesus Christ, which before was preached unto you:
Act 3:21 Whom the heaven must receive until the times of restitution of all things, which God hath spoken by the mouth of all his holy prophets since the world began.
The timing of the Second Coming of our Lord Jesus Christ is determined by and depends upon the repentance and conversion of Israel.
When our Lord Jesus Christ returns, He will set up His eternal Kingdom of God here upon this earth as taught in Luke 1:31-33,
Luk 1:31 And, behold, thou shalt conceive in thy womb, and bring forth a son, and shalt call his name JESUS.
Luk 1:32 He shall be great, and shall be called the Son of the Highest: and the Lord God shall give unto him the throne of his father David:
Luk 1:33 And he shall reign over the house of Jacob for ever; and of his kingdom there shall be no end.
Were you to become fully aware of what the Bible teaches about the promised Kingdom of God, you would understand how far astray your question about the Gospel of the Kingdom actually is. I trust that what I have written will genuinely assist your understanding of this most important subject.