Passage
When Iesus had sayd these things, he was troubled in the Spirit, and testified, and said, Verely, verely I say vnto you, that one of you shall betray me.
John 13:21 (GNV)
Nearby Context
John 13:19 From henceforth tell I you before it come, that when it is come to passe, ye might beleeue that I am he.
John 13:20 Verely, verely I say vnto you, If I send any, he that receiueth him, receiueth me, and hee that receiueth me, receiueth him that sent me.
John 13:21 When Iesus had sayd these things, he was troubled in the Spirit, and testified, and said, Verely, verely I say vnto you, that one of you shall betray me.
John 13:22 Then the disciples looked one on another, doubting of whom he spake.
John 13:23 Nowe there was one of his disciples, which leaned on Iesus bosome, whom Iesus loued.
Study Lenses
The verse centers on "Spirit", "iesus", "sayd", "things", "troubled", "testified", "said", and "verely". It is saying that the verse draws attention to "Spirit" and "iesus", so its meaning should be read from those terms before moving to application.
The nearby context moves from verse 20's "Verely verely I say vnto you If..." into verse 22's "Then the disciples looked one on another...", so "Spirit" and "iesus" belong inside that flow. In John context, the local focus is the identity of Jesus, new birth, eternal life, and belief and unbelief.
A plain takeaway is to answer the verse's own emphasis on "Spirit" and "iesus" with trust shaped by these words, not by a vague optimism outside the passage.