5 And be not disturbed by the fact that the verb is put in the future tense. For it is not said, whatsoever He has heard, or, whatsoever He hears; but, “whatsoever He shall hear, that shall He speak.” For such hearing is everlasting, because the knowing is everlasting. But in the case of what is eternal, without beginning and without end, in whatever tense the verb is put, whether in the past, or present, or future, there is no falsehood thereby implied. For although to that immutable and ineffable nature, there is no proper application of Was and Will be, but only Is: for that nature alone is in truth, because incapable of change; and to it therefore was it exclusively suited to say, “I Am That I Am,” and “You shall say unto the children of Israel, He Who Is has sent me unto you:” yet on account of the changeableness of the times amid which our mortal and changeable life is spent, there is nothing false in our saying, both it was, and will be, and is.
It was in past, it is in present, it will be in future ages. It was, because it never was wanting; it will be, because it will never be wanting; it is, because it always is. For it has not, like one who no longer survives, died with the past; nor, like one who abides not, is it gliding away with the present; nor, as one who had no previous existence, will it rise up with the future. Accordingly, as our human manner of speaking varies with the revolutions of time, He, who through all times was not, is not, and will not by any possibility be found wanting, may correctly bespoken of in any tense whatever of a verb.
The Holy Spirit, therefore, is always hearing, because He always knows: ergo, He both knew, and knows, and will know; and in the same way He both heard, and hears, and will hear; for, as we have already said, to Him hearing is one with knowing, and knowing with Him is one with being. From Him, therefore, He heard, and hears, and will hear, of whom He is; and of Him He is, from whom He proceeds.
Source: Tractates on the Gospel of John (New Advent)