In verse 18, the emphasis is on the word "all." His authority is no longer as it was when He was a man preaching in Galilee and Judea but is once again universal. It is "as it was when He was with the Father" before. He has died and been resurrected, and all authority is once again His. Therefore, His disciples are to understand that wherever they go, everything is subject to His authority. This is a good thing to remember: Everything is subject to Christ's authority.
As they go, they are to make disciples. Teaching and baptizing do not make a person a disciple, though they play a part. Just because a person is baptized does not mean he is converted. Nor does it mean he is a member of the church of God or part of the Family of God. Just because he has been taught the way of God does not mean that he has fully accepted and committed himself to what has been taught.
This is why the emphasis must be on "making disciples." Baptism and obedience to instruction will be a response a person will make who is being made a disciple.
At this point, baptism is very important because it is the outward sign of something exceedingly more important than the fact that one has been "dunked." Baptism is the outward sign of commitment—of coming under the authority of the Father and the Son. Disciples are baptized into the name of the Father, the Son, and the Holy Spirit. It is only when a person comes under or is committed to the authority of the Family of God that he is truly a disciple. This marks the difference between one who is truly a disciple and another who has only been dunked.
Once a person has been truly baptized and has truly committed himself to be under the authority of the Family of God, the issue for the disciple is continued learning as a student and loyalty as a member of the Family—as a new creation to the One he has committed himself to.