The Holy Spirit "will take from what is mine and declare it to you." Whatever that means, it has to be important, because it is repeated in the next sentence, with the explanation, "Everything that the Father has is mine."
Now, God doesn't have "possessions." God IS. So if the Holy Spirit "takes from what is mine" (as Jesus said), he is drawing from "everything that the Father has" and "declaring it" to us: revealing, offering, giving away "the life that was before the beginning" so that we can be, as St. Peter wrote, "sharers in the divine nature."