Mark 11:1-2 The beginning of the gospel of Jesus Christ, the Son of God. As it is written in the Prophets: "Behold, I send My messenger before Your face, who will prepare Your way before You."
The Gospel of our Lord Jesus Christ is the complete New Testament including the four gospels and all the epistles. These will include the everlasting gospel (message of the cross), sanctification or purification to holiness, transformation into His image, and the glorification as His bride, etc.
Everlasting Gospel (Salvation)
Luke 24:46-47 Then He said to them, "Thus it is written, and thus it was necessary for the Christ to suffer and to rise from the dead the third day, and that repentance and remission of sins should be preached in His name to all nations, beginning at Jerusalem."
Our Lord sends us to preach the everlasting gospel to all nations. The gist of the gospel of salvation (message of the cross) is as follows:-
Christ was the Word made flesh, born of the virgin Mary and was named Jesus, who was betrayed by the priesthood. He was scourged and crucified under Pontius Pilate as the sacrificial Lamb of God for the propitiation of our sins. He descended to Hades and overcame death thus setting us free from the fear of death. On the third day, He was resurrected, and the Spirit who resurrected Him will also help us to be glorified in Him.
The Apostle's Creed is a good example of the everlasting gospel.
Mat 16:20-21 Then He commanded His disciples that they should tell no one that He was Jesus the Christ. From that time Jesus began to show to His disciples that He must go to Jerusalem, and suffer many things from the elders and chief priests and scribes, and be killed, and be raised the third day. Then Peter took Him aside and began to rebuke Him, saying, "Far be it from You, Lord; this shall not happen to You!"
Though His disciples confessed Jesus as the Christ, the Son of God yet they were not saved because they only believed that He was the coming King (prosperity and blessings only), but refused to accept Him as the sacrificial Lamb for the propitiation of their sins (everlasting gospel)
Similarly all those who had confessed and accepted Jesus as the Messiah and the Son of God in evangelistic meetings, but had not believed the everlasting gospel, they may not be saved nor born again.
Mark 16:13-14 And they went and told it to the rest, but they did not believe them either. Later He appeared to the eleven as they sat at the table; and He rebuked their unbelief and hardness of heart, because they did not believe those who had seen Him after He had risen.
His disciples did not believe that Jesus came as the Lamb of God nor His resurrection even after hearing their testimonies of His resurrection, until after He personally revealed Himself to them.
John 20:20-22 When He had said this, He showed them His hands and His side. Then the disciples were glad when they saw the Lord. So Jesus said to them again, "Peace to you! As the Father has sent Me, I also send you." And when He had said this, He breathed on them, and said to them, "Receive the Holy Spirit."
After His disciples had believed His resurrection and the gospel, they were saved and only then He gave them the indwelling Holy Spirit as the seal of redemption, which is the sign of being born again.
Brethren, get your sheep to believe, understand and memorize the Apostle's Creed, which will start their long life journey with the Lord into His everlasting Kingdom.