Perfect lead-in to Good Friday, brother. The foot washing definitely goes beyond the literal, as does most aspects of our faith, and points to a much more significant cleansing where we are washed in the blood of Jesus Christ to spiritually clean our souls, yielding redemption and unity with our God. That cleansing is complete down to the lowest parts of our bodies: mind, heart, and soul.
Jesus' intercessory prayer prior to entering the garden where he would be betrayed by Judas and arrested, marking the beginning point of Jesus' suffering at the hands of the authorities, was focused largely on His disciples, and the verse you quoted from John 17:17 is outstanding:
“
Sanctify them through thy truth: thy word is truth.”
Jesus prays to the Father to set his disciples apart (i.e. sanctify) from this world and the clutches of evil and sin orchestrated by the devil. And, He says "thy word is truth." It's funny how Jesus reveals another "truth" to Pilate just prior to the beatings, torture, and ultimate crucifixion:
John 18:37-38
New King James Version
37 Pilate therefore said to Him, “Are You a king then?”
Jesus answered, “
You say rightly that I am a king. For this cause I was born, and for this cause I have come into the world, that I should bear witness to the truth. Everyone who is of the truth hears My voice.”
38
Pilate said to Him, “What is truth?” And when he had said this, he went out again to the Jews, and said to them, “I find no fault in Him at all.
Jesus answered Pilate directly by saying He
is a King whose cause is to "bear witness to the truth." Then, Pilate asks a great question, "What is truth?" All Pilate knew was life in a secular government and world where truth is an abstraction that has many different applications and interpretations. Pilate probably lived in a world of lies more than one of truth.
Pilate asked the question, but he really didn't want an answer from Jesus, and I also sense a hint of sarcasm in the question. What is truth in this train-wreck of a world? Pilate didn't wait for an answer to the question from Jesus but "went out again to the Jews" to say he didn't find any fault in Jesus,
washing his hands of the tragic event he had been forced to participate in as Roman governor of Judea.
As Jesus prayed to the Father for His disciples, one key point He makes is "Thy Word is Truth" as you pointed out, Pilgrim. The Words that come from the Father are truth, Jesus and the Father are one, so the Words of Jesus are truth as well: "In the beginning was the Word, and the Word was with God, and the Word was God" (John 1:1). God's Word, The Holy Bible, is truth, and the living Word brought us the gospel that He became human to suffer and die for our sins on the cross.
As much as I try to grasp the horror of that awful death our Lord suffered, it eludes me on the whole. The hymn, "Were You There (When They Crucified My Lord)" that will be sung at many Good Friday services today is probably the closest I can come to capturing a small piece of the emotion I should feel and the gratitude as well. Johnny Cash's rendition of this piece at the link below is quite profound:
Johnny Cash - Were You There (When They Crucified My Lord) [Live] | The Road to Nashville - YouTube