In comments made in November but only published on Friday, Francis also said priests should leave their comfort zone and get out among people on the margins of society, otherwise they may turn into "abstract ideologists".
The Italian Jesuit journal Civilta Cattolica published an exclusive text of the comments, made in a three-hour, closed-door meeting the Argentinian-born pontiff had in late November with heads of orders of priests from around the world.
"Formation (of future priests) is a work of art, not a police action. We must form their hearts. Otherwise we are creating little monsters. And then these little monsters mould the people of God. This really gives me goose bumps," he said.
Since his election in 2013 as the first non-European pope in 1,300 years, Francis has been prodding priests, nuns and bishops to think less about their careers in the Church and to listen more to the needs of ordinary Catholics, especially the poor.
Taking over an institution reeling from child sex abuse, financial and other scandals and losing members to other religions, Francis has tried to refocus on the basic Christian teachings of compassion, simplicity and humility.
His conversation with the members of the Union of Superiors General is important because they will transmit his wishes directly to priests in their religious orders around the world.
"The ghost to fight against is the image of religious life understood as an escape or hiding place in face of an 'external' difficult and complex world," he told them.
Francis said priests had to have "real contact with the poor" and other marginalized members of society.
"This is really very important to me: the need to become acquainted with reality by experience, to spend time walking on the periphery in order really to become acquainted with the reality and life-experiences of people," he told them.
"If this does not happen we then run the risk of being abstract ideologists or fundamentalists, which is not healthy."
The leader of the world's 1.2 billion Roman Catholics has set a new tone in the Vatican, rejecting the lush papal residence his predecessors used and opting for a small suite in a Vatican guest house, where he eats in the common dining hall.
Civilta Cattolica is the same periodical that ran a landmark interview with Francis in September in which he said the Church must shake off an obsession with teachings on abortion, contraception and homosexuality and become more merciful.
Francis, known as the "slum bishop" in Argentina because of his work among the poor, said reaching out to marginalized people was "the most concrete way of imitating Jesus".
His own first visits after moving to the Vatican were to a jail for juveniles and to the southern Italian island of Lampedusa to pay tribute to impoverished immigrants who have died trying to get to Europe.
Francis has said several times since his election that he feels the Vatican is too self-centered and needs to change.
A committee of eight cardinals from around the world that he has appointed to advise him on how to reform the central Vatican administration, know as the Curia, is due to submit its recommendations.