A.T. Robertson Commentary John 1:31

"And I knew him not; but that he should be made manifest to Israel, for this cause came I baptizing in water." — John 1:31 (ASV)

And I knew him not (καγω ουκ ηιδειν αυτον). Repeated in verse 33. Second past perfect of οιδα as imperfect. He had predicted the Messiah and described him before he met him and baptized him. See the Synoptics for that story. Whether John knew Jesus personally before the baptism we do not know.

But that he should be made manifest to Israel (αλλ' ινα φανερωθη τω Ισραηλ). Final clause with ινα and first aorist passive subjunctive of φανεροω. The purpose of John's ministry was to manifest to Israel with their spiritual privileges (1:49) the presence of the Messiah. Hence he was baptizing in water those who confessed their sins, he means, as in Mr 1:5. The Synoptic account is presupposed all along here.