Pell, in a missive believed to be signed by him and posted on social media on Saturday by a support group, claimed that faith was the source of his strength and suffering gave him purpose and direction, reports Efe news.
"The knowledge that my small suffering can be used for good purposes through being joined to Jesus' suffering gives me purpose and direction," said the former third-ranking Vatican official in a two-page letter dated August 1 from his prison in Melbourne.
In the message, he also expressed gratitude to the people who supported and believed in him, adding that he was waiting to find out the result of his appeal against the sentence.
A spokesperson for the Australian justice department told The Sydney Morning Herald newspaper that they were investigating whether Pell violated prison rules, as the letter somehow found its way to social media.
Pell was convicted in March after a jury found him guilty on five counts of paedophilia for sexually abusing two choir-boys at St Patrick's Cathedral in Melbourne between 1996-97.
The decision, based primarily on the victim's testimony, was reached unanimously in a second trial, after another jury had earlier failed to reach an agreement on the process involving a different victim who never reported the abuse and died of a drug overdose in 2014.
Pell's defence has presented three arguments in the appeal - one of which underlines that the jury was unable to establish guilt "beyond all reasonable doubt" - as well as evidence presented during the trial purporting to prove the cardinal could not have committed the crimes.
The other two arguments, more technical in nature, denounce that the defence was not allowed to present a 19-minute animated video seeking to reconstruct the incidents that allegedly showed how Pell could not have committed the abuses.
( With inputs from IANS )