Passage
I tell you [it] now before it happens, that when it happens, ye may believe that I am [he].
I tell you [it] now before it happens, that when it happens, ye may believe that I am [he].
John 13:17 If ye know these things, blessed are ye if ye do them.
John 13:18 I speak not of you all. I know those whom I have chosen; but that the scripture might be fulfilled, He that eats bread with me has lifted up his heel against me.
John 13:19 I tell you [it] now before it happens, that when it happens, ye may believe that I am [he].
John 13:20 Verily, verily, I say to you, He who receives whomsoever I shall send receives me; and he that receives me receives him who has sent me.
John 13:21 Having said these things, Jesus was troubled in spirit, and testified and said, Verily, verily, I say to you, that one of you shall deliver me up.
The verse centers on "tell", "before", "happens", and "believe". It is saying that the verse draws attention to "tell" and "before", so its meaning should be read from those terms before moving to application.
The nearby context moves from verse 18's "I speak not of you all I..." into verse 20's "Verily verily I say to you He...", so "tell" and "before" 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 "tell" and "before" with trust shaped by these words, not by a vague optimism outside the passage.