This might surprise some, but the unity for which our Lord prayed in John 17:21 is already in existence among those who are truly His. Let me explain what I mean.
In Scripture several types of unity are seen. There is unity between Christ and God: "I and the Father are one." (John 10:30). There is the unity that exists between Christ and His people: "I in them." (John 17:23). Then there is the unity of the faith spoken of in today's text. This unity, we must notice, is something that we are instructed to move towards 'until we all reach unity in the faith." Though it does not exist at present in some parts of the Church, we are encouraged to reach it.