Albert Barnes on John 5:31

If I bear witness of myself. If I have no other evidence than my own testimony about myself.

My witness. My testimony; my evidence, The proof would not be decisive.

Is not true. The word true. here, means worthy of belief, or established by suitable evidence. See Mt 22:16: "We know that thou art true"--that is, worthy of confidence, or that thou hast been truly sent from God, Lk 20:21; Jn 8:13,17. The law did not admit a man to testify in his own case, but required two witnesses, De 17:6. Though what Jesus said true Jn 8:13,17, yet he admitted it was not sufficient testimony alone to claim their belief. They had a right to expect that his statement that he came from God would be confirmed by other evidence. This evidence he gave in the miracles which he wrought as proof that God had sent him.

(d) "If I bear witness" Ps 27:2, Jn 8:14, Rev 3:14