John 5:36

“But the witness which I have is greater than that of John, for the works which the Father has granted me to accomplish, these very works which I am doing, bear witness to me that the Father has sent me.”

His Father has provided Him with many witnesses. Every miracle He does bears testimony to Him for it demonstrates that His Father is with Him. Indeed all to whom He speaks are witness to the signs and miracles He has done, the effectiveness of His words, the authority that He has revealed, His power over evil spirits. His life was a constant source of such things, and these very things bear witness that His Father is with Him. As one of their own had testified, ‘no one can do these signs that you do, unless God is with him' (John 3:2). So His very works prove that He is from the Father. (Note that their witness is not to the sceptic but to the religious mind wanting to know the truth. They are signs not proofs. Compare Matthew 11:2).

Others had done miracles in the past, but none had done so on the vast scale and with the completeness that He did. Nor had they like Him constantly openly faced the world of evil spirits and defeated it. He performed miracles when and where He would and they could not point to a case of one who had come for healing and had gone away unhealed or of an evil spirit that had refused to obey His command (Mark 6:5 refers to those who would not come, not to failures by Him). He was a constant revelation of the Father's power.