God: "I looked for someone to take a stand for me, and stand in the gap" (Ezekiel 22:30)
The Bible does not so much command Christians to be witnesses for Jesus as much as it identifies those who are witnesses for Jesus as Christians. As fish swim and birds fly and the IRS collects taxes, all Christians are identified as witnesses for Christ.
Failure to see Jesus Christ as the final revelation of truth is a major error that will open the door of the church to a multitude of heresies, taught in the name of truth. Every true movement initiated by the Spirit of God leads men back to the words of Christ which were inscripturated by His own inspiration.