Francis has repeatedly warned clerics against being ambitious and has curtailed the use of honorific "monsignor" titles as part of his plans to revamp the Vatican's scandal-tainted bureaucracy and finances.
"The post of cardinal is not a promotion or an honor or a decoration. It is simply a service that demands a wider view and a bigger heart," the Pope said in his letter, made public by the Vatican on Monday.
He said the 19 new cardinals, who were named on Sunday and will officially be appointed at a consistory on February 22nd, should make sure they follow the example of Jesus Christ - "the path of humility".
"You should take up the post with elation and joy but make sure this sentiment is far from any expression of worldliness, any celebrations that are foreign to the evangelical spirit of austerity, sobriety and poverty."