"Good will defeat evil, truth will defeat lies and Salvation will defeat condemnation," said the pope, adding that he has relentlessly prayed for Lula.
He also lamented the hurdles Lula has faced in the past few months, including the death of his wife Marisa Sanchez, his brother Genival Ignacio and his grandson Arthur Lula da Silva when he was only seven years old.
"I would like to express my spiritual support and encourage him to keep having hope and faith in God."
The letter delved into the pope's vision of politics, saying that "political responsibility is a constant challenge for those who receive the mandate of serving their country, protecting its residents, working in order to create the conditions of a fair and valuable future."
"Just like my predecessors, I am convinced that politics can become a prominent form of charity if the fundamental respect for life, freedom and people's dignity are being applied," he concluded.
Lula remains in prison regarding a lawsuit against him for which the Odebrecht construction company allegedly gave him a luxurious apartment in exchange for contracts with Petrobras, a state-owned oil company. Lula and the PT deny the corruption charges.
Earlier this month, the American Association of Jurists (AAJ) categorized him as a 'political prisoner.'
The AAJ said Lula's sentence "was issued as a consequence of an accusation produced by violating the due process of law, that prejudiced (his) right to a defense, and without evidence."