What does John 13:10 mean?

After objecting to the idea of Jesus washing his feet, Peter is told he has no choice. He responds by asking Jesus to cleanse his feet, head, and hands (John 13:6–9). That all but proves Peter understands that Jesus is setting up a spiritual lesson. The broader point Jesus means to teach by this act will be explained later (John 13:12). For now, Jesus explains why full-body cleaning—as part of this spiritual metaphor—is not necessary for these disciples.

As Jesus and Peter discussed washing of feet, they have used the Greek root word niptō. Here, when Jesus refers to "bathing," He uses the word louō. The difference between these two terms is almost exactly the same as between the English words "wash" and "bathe." One implies a spot-cleaning action, the other an all-over deep clean. Also noteworthy is that Jesus uses a form of louō that's past-tense, while niptō is a continuing action. The one who is bathed is "completely clean," despite needing to have their feet routinely washed. Being re-bathed is unnecessary.

The symbolism of that remark crystalizes when Jesus completes the statement: "you [Peter] are [bathed] clean, but not every one of you [who are here are bathed clean]." Verse 11 reiterates how Jesus is fully aware that Judas is a false follower (John 13:11).

Taking verses 8 through 11 together gives a potent explanation of salvation and eternal security. Those who are once-for-all washed don't need to be entirely cleansed again. That is, they have no need to be re-saved. Rather, they need to have the incidental "dirt" of their daily walk washed. Our fellowship with Christ is impacted by unconfessed sin. Those who are "clean" are those who've experienced spiritual cleansing. For such people, only the minor foot-washing is needed. But those who've never been "bathed" entirely are the unsaved—like Judas, who is "unclean."