Church Fathers on John 8:26

Our Lord having said, You believe not that I am, you shall die in your sins; they inquire of Him, as if wishing to, know in whom they are to believe, that they might not die in their sin: Then said they to Him, Who are You? For when you said, If you believe not that I am, you did not add, who you are. But our Lord knew that these were some who would believe, and therefore after being asked, Who are You? that such might know what they should believe Him to be, Jesus said to…

In some copies we find, Who also speak to you; but it is more consistent to read for (quia), not, who (qui): in which case the meaning is: Believe Me to be the beginning, for your own sakes have I condescended to these words.

See here the madness of the Jews; asking after so long time, and after all His miracles and teaching, Who are You? What is Christ’s answer? From the beginning I speak with you; as if to say, you do not deserve to hear any thing from Me, much less this thing, Who I am. For you speak always, to tempt Me. But I could, if I would confound and punish you: I have many things to say, and to judge of you.