In chapter 13 of John’s Gospel we find the story of Jesus washing the feet of the disciples, the task of a slave. He later tells them that they must do the same for each other.

Rembrandt depicts the moment when Jesus washes Peter’s feet. He protests that he will never allow Jesus to do this menial task for him. Jesus says that only in this way can Peter have a share in his life. Peter then goes over the top and asks to be washed all over. Jesus is patient but makes it clear that any idea of being better than others is precisely what he is trying to get rid of.
Rembrandt wants to show what is involved in washing feet. He is not concerned with symbolism but with the posture which a person must adopt if he is going to wash feet. Jesus is hunkered down, balanced securely, so that he can reach out towards the feet. He is not playing games but seriously focused on the job in hand. He does not glance upwards at Peter, nor backwards at the other disciples. He is doing a practical kindness, a tender service.
Perhaps Rembrandt was remembering his depiction of Jesus healing the leper, the way he abolishes the distance between sickness and health. Here, while retaining his role as teacher and Lord, he abolishes the distance between commander and commanded, teacher and pupil. There is only a sharing of life: a dusty foot and a hand that washes it.
In a few strokes Rembrandt has depicted the radical simplicity of Jesus’ gospel.