In the Lord's final instructions to His disciples, known as the Great Commission, He told His disciples "...Go ye into all the world, and preach the Gospel to every creature" (Mk. 16:15). Matthew quotes Jesus as saying, "Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost: Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you alway, even unto the end of the world" (Matt. 28:19-20). To the skeptic, these two commands appear to be different, for in the first, Jesus tells them to "preach the Gospel," while in the second, He does not mention "preaching," or "the Gospel," but speaks of "teaching," "baptizing," and again, "teaching." Why the difference?
There is a very simple explanation. In Matthew 28:19-20, the first word translated "teach" is μαθητεύω (mathēteuō), which means "to make a disciple." His second reference to "teaching" is διδάσκω (didaskō), which means "to impart instruction." We will see from Paul's epistles, it is the preaching of the Gospel that results in folks becoming Christ's disciples. So basically, Matthew and Mark are saying the same thing: "go proclaim the Gospel and make disciples."
Why then does Matthew emphasize the two additional commands to baptize and teach? Again, the answer is simple. Matthew is the only Gospel to mention the Church (16:18; 18:17). He obviously wanted his readers to know that getting souls saved was just the beginning of the Lord's commission. Once a person accepts the Gospel and becomes a disciple, that person needs to identify with the Body of Christ in baptism, and then needs to be instructed on how to go about living the Christian life. This is explained quite well in Ephesians 4:11-16. One might say that Mark focuses upon "planting," and Matthew's focus is on "pruning."
So what is this Gospel the Lord's disciples are commanded to preach? The answer is found in the writings of the Apostle Paul. We begin with his epistle to the Romans:
1:1 "Paul, a servant of Jesus Christ, called to be an apostle, separated unto the Gospel of God."
1:9 "For God is my witness, whom I serve with my spirit in the Gospel of His Son, that without ceasing I make mention of you always in my prayers."
1:15 "So, as much as in me is, I am ready to preach the Gospel to you that are at Rome also."
1:16 "For I am not ashamed of the Gospel of Christ: for it is the power of God unto salvation to every one that believeth; to the Jew first, and also to the Greek."
2:16 "In the day when God shall judge the secrets of men by Jesus Christ according to my Gospel."
10:15 "And how shall they preach, except they be sent? as it is written, How beautiful are the feet of them that preach the Gospel of peace, and bring glad tidings of good things!"
10:16 "But they have not all obeyed the Gospel. For Esaias saith, Lord, who hath believed our report?"
11:28 "As concerning the Gospel, they are enemies for your sakes: but as touching the election, they are beloved for the fathers' sakes."
15:16 "That I should be the minister of Jesus Christ to the Gentiles, ministering the Gospel of God, that the offering up of the Gentiles might be acceptable, being sanctified by the Holy Ghost."
15:19 "Through mighty signs and wonders, by the power of the Spirit of God; so that from Jerusalem, and round about unto Illyricum, I have fully preached the Gospel of Christ.
15:20 "Yea, so have I strived to preach the Gospel, not where Christ was named, lest I should build upon another man's foundation."
15:29 "And I am sure that, when I come unto you, I shall come in the fulness of the blessing of the Gospel of Christ.
16:25 "Now to him that is of power to stablish you according to my Gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began,