There are none who do not know Him

Psalm 19:1-3  "The heavens declare the glory of God; the skies proclaim the work of His hands.  Day after day they pour forth speech; night after night they display knowledge.  There is no speech or language where their voice is not heard."

Everyone knows God to one degree or another.  Some may believe that the purpose of the Great Commission is to carry the news of the existence of God to peoples all over the earth.  But as David writes above, even those unacquainted with the Gospel message have a sense of God's existence through their interaction with the physical world.

Nature proclaims the existence of a Creator even to those who don't know the full Story.  The Great Commission, then, calls us to take the Gospel to those who have a sense of, but don't personally know, God. 

When we do this, our job is not to convince someone that God exists- He has already done that through His creation.  We are simply carrying to them the revelation that will allow them to make sense of the God with whom they have already come in contact. 

The Bible reveals the full character of God and explains just how far He has taken His love for us.  Physical creation is not the end of the story, and only gives us a glimpse of what He has to offer us. The Gospel message revealed to us in the Bible completes the Story.