28:18 All power. This constitutes a clear claim to omnipotence, and therefore to deity. Jesus Christ was omnipotent Creator in the beginning (John 1:1-3), but He had laid aside His deity to become man and die for the sin of the world (Philippians 2:5-8). Now, having become Redeemer as well as Creator, He has again assumed all the attributes of God, as well as those of glorified man.
28:19 Go ye therefore. “Go ye” should actually read “As ye go.” Jesus had already, by this time, told His disciples He was sending them into all the world (John 20:21). Now He was saying that, as they go, they were to go making disciples, then baptizing them, then teaching them all they had been taught by Christ.
28:19 Holy Ghost. The Lord here explicitly teaches the doctrine of the Trinity—one name, yet three names. The three Persons are each equally and fully God, and always presented in the logical order of Father, Son, Holy Spirit. Yet there is only one God. See also the notes on Romans 1:20.
28:20 observe all things. Therefore, how can any one profess Christ while rejecting any of His teachings, (e.g., Biblical inerrancy, literal Creation, global Flood)?
28:20 with you alway. Christ has ascended back to heaven, but He is present in every believer in the person of the Holy Spirit (John 14:16-17).