10 But the more perfectly that our oblation and sacrifice corresponds to the sacrifice of Our Lord, that is to say, the more perfectly we have immolated our love and our desires and have crucified our flesh by that mystic crucifixion of which the Apostle speaks, the more abundant fruits of that propitiation and expiation shall we receive for ourselves and for others. For there is a wondrous and close union of all the faithful with Christ, such as that which prevails between the head and the other members; moreover by that mystic Communion of Saints which we profess in the Catholic creed, both individual men and peoples are joined together not only with one another but also with him, "who is the head, Christ; from whom the whole body, being compacted and fitly joined together, by what every joint supplieth, according to the operation in the measure of every part, maketh increase of the body unto the edifying of itself in charity" ( Ephesians iv, 15-16). It was this indeed that the Mediator of God and men, Christ Jesus, when He was near to death, asked of His Father: "I in them, and thou in me: that they may be made perfect in one" ( John xvii, 23).
Source: Miserentissimus Redemptor (Vatican.va)