Vatican City (CNN) -- The washing of feet was a radical act in Jesus' time. It is no less radical in the modern era, with Pope Francis' decision to break with the long-standing papal tradition of washing only priests' feet, to include women and non-Christians in the symbolic ceremony.
The controversy began on Holy Thursday last year when Pope Francis washed the feet of two women and two Muslims at a juvenile detention center in Rome. Before this, modern Popes had only ever washed the feet of 12 priests at the Vatican, during the Mass for the Last Supper.
This year, Pope Francis visited a home for the elderly and disabled, the Fondazione Don Carlo Gnocchi, to wash the feet of "12 disabled people of different ages, ethnicities and religious confessions," during a special Lord's Supper Mass, according to the Vatican.
Those chosen for the special honor included a 16-year-old boy from Cape Verde who was paralyzed in a diving accident last year, a 19-year-old man and a 39-year-old woman diagnosed with cerebral palsy, and two 86-year-olds with mobility problems.
The 78-year-old pope smiled at each of the people whose feet he washed, but clearly struggled to get up from his knees as he moved down the line; two assistants helped him to his feet. He appeared to tire towards the end of the short ceremony.
The tradition of the pontiff washing his priests' feet is based on a passage of the Bible which says that Jesus attended to his disciples at the Last Supper, saying, "If I, your Lord and Teacher, have washed your feet, you also ought to wash one another's feet." (John 13:15)
Vatican ecclesiastical rules say that only "adult males" may have their feet washed at the Mass of the Last Supper (according to the Roman Missal, 2002), following the biblical tradition that Jesus washed the feet of 12 men.
The choice of 12 priests is also symbolic of Jesus' institution of the priesthood, which according to Catholic tradition, occurred at the Last Supper.
For many years, however, bishops around the world received permission from the Vatican to wash the feet of women as well in their churches.
In fact, when the now-Pope was Archbishop of Buenos Aires, Argentina, he washed the feet of young mothers at a maternity hospital in 2005.
And it wasn't the first -- or last -- time his choice would provoke debate: In 2001, he washed and kissed the feet of 12 AIDS patients at a hospice in the Argentinian capital, and in 2008, he washed the feet of recovering drug addicts at a rehab center in the city.
But until last year, no pope had dared to go against Vatican rules and choose anyone but priests for the Holy Thursday event. In choosing to change the practice, Francis is being as radical as Jesus was in his own time.
"According to the Talmud, the washing of feet was forbidden to any Jew except those in slavery," the US Conference of Catholic Bishops says in its Holy Thursday Mandatum.
"Jesus' action of washing the feet of his disciples was unusual for his gesture went beyond the required laws of hospitality (washing of hands) to what was, in appearance, a menial task."
By eschewing traditional practice, Pope Francis is emphasizing the spirit rather than the letter of the law -- something Jesus himself did by breaking with Jewish tradition in washing his disciples' feet.
And the spirit, or meaning, behind washing the feet of another person is one of humility and service -- that through the act, the leader becomes a servant to his followers.
The Vatican's Congregation for Divine Worship says that foot-washing "represents the service and charity of Christ, who came 'not to be served, but to serve.'"
According to Vatican spokesman Father Thomas Rosica, Francis' gesture in 2013 was one of humility and service, intended to "embrace those on the fringes of society."
From the beginning of his papacy, the pope himself has been on the "fringes" of long-standing Vatican protocol, for example by choosing to live outside of the papal palace and eschewing traditional papal garments and modes of transport.
In breaking the rules of foot-washing at the Vatican, the Pope is acknowledging what has been a practice in local churches for some time, and also reminding Catholics that the important thing is not whose foot is being washed, but the spirit behind that gesture.
CNN'sDaniel Burke and Bryony Jones contributed to this report.