Verily, verily, I say to thee, We speak that which we know, and testify that which we have seen. Lucke and Meyer think that our Lord here merely uses the pluralis majestaticus—uses it as St. Paul does, when clearly he was referring to himself alone.
It is difficult to believe this in the curious and impressive change of person here adopted, and the return to the first person singular in John 3:12. There was some reason why Jesus, in making this particular saying, uses the plural.