What does John 13:17 mean?

Several places in Scripture explain that people are only held accountable for what they know and understand. However, people are held absolutely accountable. Nature and human experience provide more than enough evidence for people to acknowledge the basic existence of God (Romans 1:18–29; Psalm 19:1). Those who sincerely seek after God will find Him (Matthew 7:7–8). There are fine points on which even sincere people can be wrong (John 9:40–41)—and obvious points on which the well-informed can be stubborn (John 5:39–40).

In other words, God does not require a person to know everything, but He expects everything a person knows to be obeyed. The same applies to the teaching Jesus has given over the last few verses. By washing the feet of the disciples, Jesus has destroyed any claims a person can have to be "too good" to humbly serve others. Human nature vainly says lesser tasks are for lesser people, and the lowly should serve the superior. Jesus' teaching by example is that humility and service are the obligation of all Christians. Those who understand this are blessed by obeying it.