He told the priests: "It's not with the cudgel [a weapon] of judgment that we will succeed in bringing the lost sheep back to the fold."
The Holy Year of Mercy is encouraging a more forgiving mentality in the Catholic church.
Around 700 priests from around the world gathered to listen to the pontiff who stressed that they must welcome people into their church and told priests to act more kindly upon those seeking forgiveness during confession.
He said that many people abandoned confession because priests had previously told them off or asked for intimate details.
Pope Francis said confessors "aren't called to judge, with a sense of superiority" but instead welcome penitents with "open arms".
Pope Francis opens the Holy Year of Mercy in December 2015.
The special group of around 1,000 confessors, known as the Missionaries of Mercy, will travel around to world to places including the United Arab Emirates, China and Burundi to minister to the world's 1.2 billion Catholics.
One priest is evening travelling to the far Canadian north and another will use a camper van to reach Catholics living in remote parts of the Australian countryside.
The pope even joked: "If you're going to the Arctic, bundle up!"
He was speaking as Ash Wednesday begins, the start of the annual church period stressing penitence before Easter.
During this special year, the Missionaries of Mercy will be able to absolve grave and relatively uncommon sins that normally must be dealt with by bishops or officials at the Vatican, such as abortion.
In Catholic teaching, abortion triggers automatic excommunication when the person is aware of the penalty and commits the sin nevertheless.
The Holy Year began on December 8th 2015 and will end on November 20th 2016.
