That all may be one. That prayer will be fully realized in us when the perfect love with which God has first loved us has passed into the very movement of our heart in fulfillment of that prayer of the Lord; and this prayer of the Lord, we believe, will not be in vain. This will come about when all our love, all our effort, all our search, all that we live and all that we speak, all that we breathe, will be God alone; when the present unity of the Father with the Son, and of the Son with the Father, will have passed into our soul and into our heart. This means: when, imitating the true and indestructible charity with which he loves us, we too shall be united with him by a continual and unalterable charity, so thoroughly attached to him that all our breathing, all our thinking, and all our speaking will be nothing but him.
In this way we shall reach the end about which the Lord in his prayer desired to be fulfilled in us: That all may be one as we are one, I in them and you in me, that their unity may be complete, and Father, all those you gave me I would have in my company where I am.
It is to this that we are destined, and it must be the goal of all our efforts: to have the grace of possessing, already in this life, the likeness of the future beatitude and a kind of foretaste of the life and the glory of heaven while still in his mortal flesh.
John Cassian (AD 360-435), Conference X, 6-7: PL 49, 827-828