Scripture puts it this way: "From such, turn away."
If we're to follow Christ, then we are to live among and relate to the unsaved, not hole up and shut ourselves off from society. Live by example, and do not condemn those around us. Tell them what we believe, fine, but do not condemn.
Jesus did not condemn the woman at the well, but told her to go and sin no more. Compassion and kindness - that is how we should witness.