Church Fathers on John 3:34

Having said, And what he has seen and beard, that he testifies, to prevent any from supposing, that what he said was false, because only a few for the present believed, he adds, And no man receives his testimony; i.e. Only a few; for he had disciples who received his testimony. John is alluding to the unbelief of his own disciples, and to the insensibility of the Jews, of whom we read in the beginning of the Gospel, He came to His own, and His own received Him not.

Or thus; There is a people reserved for the wrath of God, and to be condemned with the devil; of whom none receives the testimony of Christ. And others there are ordained to eternal life. Mark how mankind are divided spiritually, though as human beings they are mixed up together: and John separated them by the thoughts of their heart, though as yet they were not divided in respect of place, and looked on them as two classes, the unbelievers, and the believers. Looking to the…

Or, Has put to his seal, i.e. has put a seal on his heart, for a singular and special token, that this is the true God, Who suffered for the salvation of mankind.