Albert Barnes on Matthew 26:21-24

As they did eat, etc. The account contained in these verses is also recorded in Mk 14:18-21, Lk 22:21-23, Jn 13:21-22. John xiii. 21, 22. John says, that before Jesus declared that one of them should betray him, he was troubled in spirit, and testified. That is, he felt deeply the greatness of the crime that Judas was about to commit, and anticipated with much feeling the sufferings that he was to endure.

He testified. He bore witness, or he declared.