Adam Clarke on John 7:18

He that speaketh of himself, etc. - I will give you another rule, whereby you shall know whether I am from God or not: If I speak so as to procure my own glory, to gratify vanity, or to secure and promote my secular interests, then reject me as a deceiver and as a false prophet. But if I act only to promote the glory of God, to induce all men to love and obey him; if I propose nothing but what leads to the perfection of his law, and the accomplishment of its ordinances, you cannot help acknowledging me at least for a true prophet; and, if you add to this the proofs which I have given of my mission and power, you must acknowledge me as the mighty power of God, and the promised Messiah.

And no unrighteousness is in him - Or, there is no falsehood in him: so the word αδικια should be translated here; and it is frequently used by the Septuagint for שקר sheker, a lie, falsehood, etc. See in Psa 52:3; Psa 119:29, Psa 119:69, Psa 119:104, Psa 119:163; Psa 144:8. This is its meaning in Rom 2:8; where αδικια, falsehood, is put in opposition to αληθεια, truth.