This is, in my opinion, the best definition of the Gospel there is. It is concise (only 100 words) and easy to memorize. I would advise you to print out a number of copies and place it in your Bible, tape it on your mirror, put it in your car, post it on your computer, etc.
This Gospel definition was taken from the book, The Discipleship Gospel by Bill Hull & Ben Sobels. I would highly recommend this book to you. In the book they explain each piece of the definition in detail showing you how they Biblically came up with it.
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The Gospel is: the Kingdom of God has come through Jesus of Nazareth. He is Christ, the King, God’s one and only Son. He died on the cross for our sins, was buried, and was resurrected on the third day according to the Scriptures. In His great love and by His amazing grace, God our Father saves everyone who repents of their sin, believes the Gospel, and follows Jesus in the power of the Holy Spirit. When King Jesus returns on the last day, the great Day of Judgment, everyone who followed Him will enter the eternal Kingdom of God.
(Mark 1:14-18; 8:27-31; 1 Cor. 15:1-5; John 3:16; Eph. 2:8-10; Matt. 25:31-46)
from The Discipleship Gospel by Bill Hull & Ben Sobels