Be one in love
When God shall be all our love, and every desire and wish and effort, every thought of ours, and all our life and words and breath
[Abba Isaac said:] “Then will be perfectly fulfilled in our case that prayer of our Saviour in which He prayed for His disciples to the Father saying "that the love wherewith Thou lovedst Me may be in them and they in us;" (Jn 17:26) and again: "that they all may be one as Thou, Father, in Me and I in Thee, that they also may be one in us," (Jn 17:21) when that perfect love of God, wherewith "He first loved us" (1 Jn 4:19) has passed into the feelings of our heart as well, by the fulfilment of this prayer of the Lord which we believe cannot possibly be ineffectual. And this will come to pass when God shall be all our love, and every desire and wish and effort, every thought of ours, and all our life and words and breath, and that unity which already exists between the Father and the Son, and the Son and the Father, has been shed abroad in our hearts and minds, so that as He loves us with a pure and unfeigned and indissoluble love, so we also may be joined to Him by a lasting and inseparable affection, since we are so united to Him that whatever we breathe or think, or speak is God, since, as I say, we attain to that end of which we spoke before, which the same Lord in His prayer hopes may be fulfilled in us: "that they all may be one as we are one, I in them and Thou in Me, that they also may be made perfect in one" (Jn 17:23-24).
St John Cassian, Conferences X,7