Rome (CNN) Pope Francis opened the Holy Doors of St. Peter's Basilica on Tuesday, performing a ritual that has been part of the Catholic Church since the 1500s.

Holy Doors are only opened during a special year designated by a pope called a Jubilee Year.

This iteration will be a Jubilee of Mercy. The last Jubilee Year was in 2000.

More than 50,000 people attended the ceremony, including Italian Prime Minister Matteo Renzi, Italian President Sergio Mattarella and members of the Belgian royal family, according to the Vatican's official news agency.

Francis' predecessor, Pope Emeritus Benedict XVI, followed him through the Holy Doors of St. Peter's Basilica. The 88-year-old, rarely seen in public since his resignation in 2013, walked with a cane and the help of longtime aide.

May the Jubilee of Mercy bring us all closer to God's goodness and love!

"Salvation is offered to every human, to every people, without exception, to each of us," Pope Francis said during the ceremony, according to Vatican Radio . "None of us can say, 'I am holy, I am perfect, I am already saved.' "

Rules rewritten

Traditionally, there are holy doors only in the four basilicas of Rome: St. Peter's, St. John Lateran, St. Mary Major and St. Paul Outside the Walls.

But Pope Francis has rewritten the rules and said that every Catholic cathedral in the world may designate a holy door to be opened for a year, so that even those who can't come to Rome can participate in the church's Jubilee Year of Mercy by walking through the doors.

Until 1975, the Holy Doors in Rome were enclosed by a cement wall that the pope broke down using a hammer. When cement fragments fell too close to Pope Paul VI during the opening of the Holy Door on Christmas Eve in 1974, this practice was abandoned, and now bronze doors have replaced the wall.

Holy Doors symbolically represent Jesus, who said, "I am the door." (John 10:7).

What happens when Catholics walk through Holy Doors?

Walking through the Holy Doors means that you receive an indulgence, which is a lessening of the consequences attached to sin.

According to the Catholic Church, when you sin, you must go to confession and you are forgiven. But forgiveness only applies to the guilt of your sin; there may still be consequences of your sin that you may have to pay for in this life or after you die. An indulgence is a way to lessen that penalty.

As an analogy, if someone commits a crime, he is sentenced to jail time as punishment. He may be sorry for his crime and apologize, but he still must serve his sentence and deal with the consequences of his crime.

To receive a full indulgence (called a plenary indulgence), you must:

1. Walk through the Holy Doors.

2. Go to confession.

3. Receive communion.

4. Pray for the intentions of the pope.

Special sins

In another first, Francis will directly appoint special missionaries from all over the world who will receive from the pope the power to forgive sins usually reserved to the Holy See. Those sins are:

1. Desecration of the Eucharist.

2. Absolution of accomplice in a sin against the sixth commandment.

3. Ordination of a bishop without the pope's approval.

4. Violation of the sacrament of confession (a priest divulging what he has been told in confession).

5. Physical violence against the Roman Pontiff