[John 17:21-23 ESV] 21 that they may all be one, just as you, Father, are in me, and I in you, that they also may be in us, so that the world may believe that you have sent me. 22 The glory that you have given me I have given to them, that they may be one even as we are one, 23 I in them and you in me, that they may become perfectly one, so that the world may know that you sent me and loved them even as you loved me.
The reason for this "oneness" that Jesus speaks of here to His disciples is to solidify a BOND in His Kingdom. The unity of heaven has now come to earth with the sacrificial death of the Savior and the sending of His Holy Spirit to indwell (or take up residence) in the believer. This union, simply put, is the glory (shining radiance) that God gives His children. If the lost world sees this fusion of God and fellow believers living together in love, it will lead them to believe in the goodness of God and awakening occurs. It goes along with the new commandment that Christ gave (John 13:34-35). This is how we become "perfectly one."
*Life Point* "When love is in the house, the house is packed"- Toby Mac