John Frame says the following in the introduction to his book, Apologetics to the Glory of God:

...the apologist must 'set apart Jesus as Lord'...[and] his argument must presuppose that lordship. Our argument must be an exhibit of that knowledge, that wisdom, which is based on the 'fear of the Lord,' not an exhibition of unbelieving foolishness. Therefore, apologetic argument is no more neutral than any other human activity. In apologetic argument, as in anything else we do, we must presuppose the truth of God's Word. We either accept God's authority or we do not, and not to do so is sin. It doesn't matter that we sometimes find ourselves conversing with non-Christians. Then too—perhaps especially then (for then we are bearing witness)—we must be faithful to our Lord's revelation.

To tell the unbeliever that we can reason with him on a neutral basis, however that claim might help attract his attention, is a lie. Indeed, it is a lie of the most serious kind, for it falsifies the very heart of the gospel—that Jesus Christ is Lord. For one thing, there is no neutrality. Our witness is either God's wisdom or the world's foolishness. There is nothing in between. For another thing, even if neutrality were possible, that route would be forbidden to us.

Frame's conviction in this matter is apparent in his writings, and I have been privileged to witness that conviction in his teaching as well. This understanding functions as the basis from which our apologetic must begin. There simply is no other way.