Where is the imperative in this verse? It is in the word “make disciples”, those that believe and by faith follow in obedience. The method of making disciples of all nations (“ethnos”) is through baptizing them and teaching them. As we look at the Book of Acts, this is exactly what the apostles did after the Holy Spirit came. The prime example is Acts 2:38 where Peter preached repentance – the starting point for every disciple.
More will be written tomorrow about baptism.