Albert Barnes Commentary John 8:26

"I have many things to speak and to judge concerning you: howbeit he that sent me is true; and the things which I heard from him, these speak I unto the world." — John 8:26 (ASV)

I have many things to say. There are many things that I might say to reprove and expose your pride and hypocrisy. By this he implied that he understood well their character, and that he was able to expose it. This, indeed, he had shown them in his conversations with them.

And to judge of you. To reprove in you. There are many things in you that I might condemn.

But he that sent me is true. Is worthy to be believed, and his declarations about men are to be credited. The meaning of this verse may be expressed this way:

"I have indeed many things to say blaming or
condemning you. I have already said many such things,
and there are many more that I might say; but I
speak only those things that God has commanded. I
speak not of myself; I come to execute his commission,
and he is worthy to be heard and feared. Let it not be
thought, therefore, that my judgment is rash or harsh.
It is what God has commanded."

(Compare John 7:28 regarding "he that sent me".)