Holy Gospel of Jesus Christ according to Saint John 17:20-26.

Lifting up his eyes to heaven, Jesus prayed saying: "I pray not only for them, but also for those who will believe in me through their word, 
so that they may all be one, as you, Father, are in me and I in you, that they also may be in us, that the world may believe that you sent me. 
And I have given them the glory you gave me, so that they may be one, as we are one, 
I in them and you in me, that they may be brought to perfection as one, that the world may know that you sent me, and that you loved them even as you loved me. 
Father, they are your gift to me. I wish that where I am they also may be with me, that they may see my glory that you gave me, because you loved me before the foundation of the world.
Righteous Father, the world also does not know you, but I know you, and they know that you sent me. 
I made known to them your name and I will make it known, that the love with which you loved me may be in them and I in them."

Commentary of the day 
Saint Peter Damian (1007-1072), hermit then Bishop, Doctor of the Church 
Opuscule 11 “Dominus vobiscum”, 6 (Migne 1992, p.22 rev.)

"May they all be one, as you, Father, are in me and I in you"

Holy Church, although diverse in multiplicity of persons, is brought into unity by the fire of the Holy Spirit. If, from the physical point of view, she seems to be divided among several families, yet the mystery of her profound unity loses nothing of its integrity: “because the love of God has been poured out into our hearts through the Holy Spirit that has been given to us,” (Rom 5:5). There is no question that this Spirit is both one and many at the same time: one at the core of its majesty; many in the gifts and charisms granted to the Holy Church filled by his presence. And it is this same Spirit that enables the Church to be at one and the same time single in its universal extent yet wholly present in each of its members... 

So if those who believe in Christ are one, no matter where any particular one of them happens physically to be, the whole body of the Church is there through the sacramental mystery. And everything suitable to the whole body seems suitable to each one of its members... Hence it is that, when several of the faithful are together, they can say: “Incline your ear, O Lord; answer me, for I am afflicted and poor. Keep my life, for I am devoted to you” (Ps 86[85]:1-2). And when we are alone, we can still sing: “Let us all sing joyfully to God our strength; acclaim the God of Jacob” (Ps 81[80]:2). It is not misplaced for us all to say together: “I will bless the Lord at all times; his praise shall be ever in my mouth” (Ps 34[33]:2) nor, when I find myself alone, to proclaim: “Glorify the Lord with me, let us together extol his name” (v.4) and many other, similar expressions. Solitude prevents nobody from speaking in the plural while the mass of the faithful can just as well express themselves in the singular. The Holy Spirit's power, which dwells in each of the faithful and encircles them all, means that in the latter case there is a peopled solitude and in the former a great many who form but one.