John 15:26

But the work of the Apostles was not to be wholly fruitless, nor was their experience to be wholly comprised in fruitless persecution. Ὅταν δὲ ἔλθῃ … περὶ ἐμοῦ. The Spirit of Truth will witness concerning me. The Spirit is here designated, as in John 14:16, “the Paraclete,” and the Spirit of Truth. There, and in John 14:26, it is the Father who is to give and send Him in Christ's name: here it is ὃν ἐγὼ πέμψω παρὰ τοῦ πατρός, as if the Spirit were not only dwelling with the Father, but could only be sent out from the Father as the source of the sending. This is still further emphasised in the added clause, ὃ παρὰ τοῦ πατρὸς ἐκπορεύεται. To define the mode of being of the Spirit, or His essential relation to the Father, would have been quite out of place in the circumstances. These words must be understood of the mission of the Spirit. What the disciples needed to know was that He came out from the Father, and of this they are here assured. ἐκεῖνος μαρτυρήσει περὶ ἐμοῦ, “He,” that person thus elaborately described, who is truth and who comes out from Him who sent me, “will witness concerning me”.