John 5:36
But the testimony that I have is greater than that of John. For the works that the Father has given me to accomplish, the very works that I am doing, bear witness about me that the Father has sent me.

Hebrews 12:1
Jesus, Founder and Perfecter of Our Faith
Therefore, since we are surrounded by so great a cloud of witnesses, let us also lay aside every weight, and sin which clings so closely, and let us run with endurance the race that is set before us,

Living a life of faith is our greatest witness.

Preach the gospel wherever you go, and if you must use words.