All that my Father has is mine is related closely to the thought of 3.35 and 5.20 (see there). This same claim is repeated at 17.10. The specific reference here is to the truth which Christ communicates, and this fact is made clear in the second half of the verse.

The clause introduced by that is why I said that qualifies the content of what is referred to by all that the Father has is mine. The Spirit will take the full truth that Christ gives him and tell it to the disciples.

The Spirit will take what I give him is literally “… he receives from what is mine.” Good News Translation makes the pronominal reference “he” explicit as a reference to the Spirit; it also takes the Greek present tense (“receives”) as the equivalent of the future in the previous verse, since the change of tense apparently carries no significance. Finally, it restructures the words “he receives from what is mine” to read will take what I give him. What is taken and received is, of course, the revelation. Accordingly, one may translate in some languages “the Spirit will listen to what I tell him and then he will tell it to you.”

Quoted with permission from Newman, Barclay M. and Nida, Eugene A. A Handbook on the Gospel of John. (UBS Handbook Series). New York: UBS, 1980. For this and other handbooks for translators see here .