Passage
For He knew the one who was betraying Him; for this reason He said, “Not all of you are clean.”
John 13:11 (LSB)
Nearby Context
John 13:9 Simon Peter said to Him, “Lord, not only my feet, but also my hands and my head.”
John 13:10 Jesus said to him, “He who has bathed needs only to wash his feet, but is completely clean; and you are clean, but not all of you.”
John 13:11 For He knew the one who was betraying Him; for this reason He said, “Not all of you are clean.”
John 13:12 So when He had washed their feet, and taken His garments and reclined at the table again, He said to them, “Do you know what I have done to you?
John 13:13 You call Me Teacher and Lord; and you are right, for so I am.
Study Lenses
The verse centers on "knew", "betraying", "reason", "said", and "clean". It is saying that the verse draws attention to "knew" and "betraying", so its meaning should be read from those terms before moving to application.
The nearby context moves from verse 10's "Jesus said to him He who has..." into verse 12's "So when He had washed their feet...", so "knew" and "betraying" 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 "knew" and "betraying" with trust shaped by these words, not by a vague optimism outside the passage.