The Gospel includes all truth. The Plan of Salvation is what we may call, the Blueprint of the Gospel. It is a simple drawing. All may read and understand it who have a desire to know the Mind and Will of God.
Christ, Himself, gives the key to its great meaning: "That I came into the world to do the will of My Father..." (v. 13) And, you will remember that among His last words when His mission unto the Jews was about ended: "Father, if Thou be willing, remove this cup from Me: nevertheless, not My will, but Thine be done." (Luke 22:42)
Obedient, Jesus Christ, the Eternal Son of God, came to Earth to fulfill a mission His Father sent Him to perform. We may not comprehend the metaphysics of Why?, but we do understand that such a mission as was Christ's became necessary to further the Designs of Providence. A price was demanded to Redeem mankind from the effects of Adam's Fall. What was that price? God alone could pay the price. Jesus Christ, the Lord God, paid it; The Great Redeemer died. The reason for it being, the Savior explains: "And for this cause have I been lifted up; therefore, according to the power of the Father I will draw all men unto Me, that they may be judged according to their works." (v. 15)