The Teacher's example
When Jesus washed his disciples’ feet he was doing far more than giving them an example of humble service. It was an acted parable of what he was about to do for them on the cross, as his comments show. Not all of them were clean, he said, referring to Judas (10), though Judas too had had his feet washed. Only afterwards would they be able to understand the primary meaning of what he had done for them (7).
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Yet it is an example for them too, and this much they should have been able to understand already (12). They recognised him as Teacher and Lord and they were right to do so (13). So if he who is Lord over all was willing to humble himself so greatly, how much more should those who follow him be willing to humble themselves to “wash one another’s feet” (14).
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This doesn’t refer to a literal act but means in humility counting others better than ourselves, looking out for their interests i.e. “your attitude should be the same as that of Jesus Christ” (Phil.2:3-5). This is the very reason why Paul quoted the famous verses in Phil.2:5-11, as an illustration of how Christians should behave towards one another. Such an attitude involves costly action, just as the cross was costly for Jesus.