“He will bear witness on my behalf. You must bear witness as well.” John 15:26-27.16:1-4a.

Jesus said to his disciples: "When the Advocate comes whom I will send you from the Father, the Spirit of truth that proceeds from the Father, he will testify to me.
And you also testify, because you have been with me from the beginning.
I have told you this so that you may not fall away.
They will expel you from the synagogues; in fact, the hour is coming when everyone who kills you will think he is offering worship to God.
They will do this because they have not known either the Father or me.
I have told you this so that when their hour comes you may remember that I told you."

“He will bear witness on my behalf. You must bear witness as well.”

The Joint Mission of the Son and the Spirit: …When the Father sends his Word, he always sends his Breath. In their joint mission, the Son and the Holy Spirit are distinct but inseparable. To be sure, it is Christ who is seen, the visible image of the invisible God (cf. Col 1:15), but it is the Spirit who reveals him.

Jesus is Christ, “anointed,” because the Spirit is his anointing, and everything that occurs from the Incarnation on derives from this fullness (cf. Jn 3:34). When Christ is finally glorified (cf. Jn 7:39), he can in turn send the Spirit from his place with the Father to those who believe in him: he communicates to them his glory (cf. Jn 17:22), that is, the Holy Spirit who glorifies him (cf. Jn 16:14). From that time on, this joint mission will be manifested in the children adopted by the Father in the Body of his Son: the mission of the Spirit of adoption is to unite them to Christ and make them live in him…

The mission of Christ and the Holy Spirit is brought to completion in the Church, which is the Body of Christ and the Temple of the Holy Spirit. This joint mission henceforth brings Christ’s faithful to share in his communion with the Father in the Holy Spirit. The Spirit prepares men and goes out to them with his grace, in order to draw them to Christ. The Spirit manifests the risen Lord to them, recalls his word to them and opens their minds to the understanding of his Death and Resurrection. He makes present the mystery of Christ, supremely in the Eucharist, in order to reconcile them, to bring them into communion with God, that they may “bear much fruit.” (Jn 15:5).

Thus the Church’s mission is not an addition to that of Christ and the Holy Spirit, but is its sacrament: in her whole being and in all her members, the Church is sent to announce, bear witness, make present, and spread the mystery of the communion of the Holy Trinity.