John Lightfoot on John 8:13

[Thou bearest record of thyself.] This and the following passages uttered in dispute, whether Christ was the light or no, bring to mind what was wont to be transacted amongst them in their witnessing about the appearance of the new moon. We have it in Rosh Hashanah.

I. It was to be attested before the Sanhedrim by two persons that they saw the new moon. So Christ mentions two witnesses attesting him to be the light, viz. the Father and himself, verse 18.

II. They did not allow the testimony about the new moon, unless from persons known to the Sanhedrim: or if they were unknown, there were those sent along with them from the magistracy of that city where they lived, that should attest their veracity. Compare verses 18, 19: "I bear witness of myself, and ye know me not. My Father also bears witness of me; but ye have not known my Father."

III. One witness is not to be believed in his own cause. So the Pharisees, verse 13, "Thou bearest record of thyself; thy record is not true."

IV. The father and the son, or any sort of relatives, are fit and credible witnesses: verse 18; "I am one that bear witness of myself, and the Father that sent me beareth witness of me."