John Paul's reflections were written during his 2 1/2-week hospitalization at Rome's Gemelli Polyclinic, where he was taken for the second time in a month because of breathing problems.
He was discharged Sunday and is convalescing at his apartment with a view of St. Peter's Square. Accepting the advice of his doctors, John Paul's only Holy Week commitment is an Easter Sunday blessing.
The 84-year-old pontiff has designated other prelates to stand in for him, with Cardinal Camillo Ruini, his vicar for Rome, scheduled to preside over the Palm Sunday procession and Mass in St. Peter's Square.
Vatican officials said John Paul is expected to appear Palm Sunday at his window overlooking the square, but it was not known if he planned to speak.
John Paul has only been heard speaking publicly twice - each time just a few sentences delivered in a raspy voice - since undergoing throat surgery Feb. 24 to ease a breathing crisis. He also suffers from Parkinson's disease and crippling hip and knee ailments.
The letter released Friday contained his most personal comments on his fragile health since Feb. 1, when he was rushed to the hospital for the first time last month.
"My thoughts turn to you, dear priests, as I spend this time recuperating in hospital, a patient alongside other patients, uniting in the Eucharist my own sufferings with those of Christ," John Paul wrote.
"In this spirit I want to reflect with you on some aspects of our priestly spirituality," the letter said.
"All the world followed the suffering of the Holy Father and how his thoughts went out to all," said Cardinal Dario Castrillon Hoyos, head of the Vatican's office for priests, in introducing the message at a Vatican news conference.
The message included a call for priests to inspire others to become clergy by setting both a "joyful" and "impassioned" example.
"A priest `won' by Christ more easily `wins' others," the message said.
A gaunt John Paul appeared in a live video shown Thursday evening to hundreds of young people who filled a Rome basilica to pray for him.
In an appearance lasting less than a minute, the pope made the sign of the cross to the crowd, which erupted in cheers. At one point, John Paul covered his eyes with his hand, and the live video signal cut out. The pontiff did not speak.
John Paul has traditionally celebrated Holy Thursday Mass, when thousands of priests fill St. Peter's Basilica to renew their vows. This year, Cardinal Giovanni Battista Re, head of the Vatican's office for bishops, will preside.
The pope will also skip another ritual that day, the washing of the feet of 12 priests to commemorate Christ's Last Supper with his apostles before he was crucified.
John Paul has not named anyone for the Way of the Cross procession at the Colosseum on Good Friday, and it was not known whether he might participate in that day's events.
By Victor L. Simpson