The Great Commission (Matthew 28:18-20; Luke 24:44-49; John 20:21; Acts 1:8) is about Doctrine and Theology.
“to disciple all nations” means the nations have to be taught the deeper truths of discpleship.
“to teach them” means teaching and theology. We have to teach them about the gospel, and who God is, who Christ is, who the Holy Spirit is, what the doctrine of the Trinity means, the atonement, the resurrection, the meaning of sin and repentance, what is the church, etc.
I have seen Paul Washer make statements like these before, and it got me thinking and I agree. Very good article.
As I have time, I want to explore this issue more. The blog by a guy named Drew Mery that I linked to here looks very good, from just looking at it for a few minutes today.