Answer:
This simple act was to show that unless they be washed away of their sins, they cannot inherit the kingdom of God. The message of repentance and forgiveness was at the very heart of Christ's teachings. In Matthew 6 Jesus said this immediately after giving us the Lord's Prayer.
The early Christian church introduced the custom to imitate the humility and selfless love of Jesus, who washed the feet of the Twelve Apostles at the Last Supper (John 13:1–15), the night before his Crucifixion.
