Separation from the World as Witness to the World

Consider our Lord's High-Priestly Prayer:

John 17: "Father, the hour has come. Glorify Your Son, that Your Son also may glorify You, as You have given Him authority over all flesh, that He should give eternal life to as many as You have given Him. ... I have manifested Your name to the men whom You have given Me out of the world. ... I pray for them. I do not pray for the world but for those whom You have given Me, for they are Yours. ... Holy Father, keep through Your name those whom You have given Me, that they may be one as We are. ... I have given them Your word; and the world has hated them because they are not of the world, just as I am not of the world. I do not pray that You should take them out of the world, but that You should keep them from the evil one. They are not of the world, just as I am not of the world. Sanctify them by Your truth. Your word is truth. As You sent Me into the world, I also have sent them into the world. ... I do not pray for these alone, but also for those who will believe in Me through their word; that they all may be one, as You, Father, are in Me, and I in You; that they also may be one in Us, that the world may believe that You sent Me. And the glory which You gave Me I have given them, that they may be one just as We are one: I in them, and You in Me; that they may be made perfect in one, and that the world may know that You have sent Me, and have loved them as You have loved Me. ..."

One thing we see in this passage is that we are sent into the world, just as our Lord was sent from Heaven into the world.  We are not of the world, just as he is not of the world.  When our mission is over, we will return to be with our Lord, just as he has returned to Heaven to be with the Father.  Therefore, our tour through this world is a mission to call men into the fellowship of the Father and the Son, just as was our Lord's mission into this world.

The witness to the world is not that we love the world (though we do), nor is it that the Father loves the world (though he does).  Our mission is characterized in this passage as witnessing to the world that God sent Christ into the world.  The proof of this is that his followers bear within themselves the love of God for one another which transcends all knowledge and natural human capacity.

The Church's internal fellowship of love, witnessed by the world, is the divine apologetic for the mission of Christ and the truth of the Gospel.

The implication of this is that it is our 'separation' from the world and our allegiance to one another in the church -- in the right manner -- which is the saving testimony to the world.