We are called to witness to people whether they are atheists, agnostics, or deluded by a false religion. Jehovah’s Witnesses deny that Jesus is the Son of God, and therefore must think He is a liar when He claimed that He was not only the Son of God, but the only way to the Father Himself. From their own text, even if terribly misguided and false, there is still enough truth there to lead them to an understanding that Jesus is the Son of God, the Messiah, and the only way of salvation.
