It is another that witnesseth concerning me; and £ I know that the witness which he witnesseth concerning me is true. It is a mistake, with Ewald, De Wette, and many others, to suppose that this refers to the testimony of John the Baptist.
By Augustine, Hengstenberg, Luthardt, Godet, Meyer, etc., it has been perceived that the "other" ( ἄλλος) refers to the Father. Jesus expressly declines to receive John's testimony as his justification or sufficient vindication, and he contrasts it with the higher confirmation which in three distinct ways is already and continuously vouchsafed to him.
The present tense, μαρτυρεῖ, is in striking contrast to the testimony of John already silenced by imprisonment or death. The methods of this testimony are subsequently analyzed and described.