In 2012, the 46-year-old actor and his 49-year-old ex-wife Anna Faris' boy, Jack, was born nine weeks prematurely and spent a month in intensive care. His high-risk start to life made Pratt feel "compelled" to talk about his faith, which stems from his childhood.
The Guardians of the Galaxy star has just been to Rome, where he filmed a documentary on St. Peter and met Pope Leo.
"I was baptised in a Catholic Church and have believed in God since childhood," Pratt wrote in a personal essay for Maria Shriver's The Sunday Paper.
"I attended various religious institutions, including Catholic, Lutheran and Evangelical churches. I knew little about the distinctions between denominations, but so much about the healing power of Grace.
"Mine was a broken road marked by cycles of sin and forgiveness; a pattern of rebellion, shame and grace on repeat, leading to increased extremes of emotional highs and lows.
"Then, when my son Jack was fighting for his life, I found myself back on my knees, pleading to God for a miracle. I promised that if Jack survived, I would devote my life to sharing God's message. This time I meant it."
Jack is now "healthy", and Pratt - who is also dad to daughters Lyla, five, and Eloise, three, and son Ford, 13 months, with his 36-year-old spouse, author Katherine Schwarzenegger - thanks God every day for saving his eldest son's life, as well as for the existence of his other children.
The star - who described the trip to Rome as a "profound experience", which allowed him to delve deeper into exploring his faith - added that he knew how fortunate he is.
"I know there are countless parents who pray those same prayers and don't receive the answer I did. My heart is with them always."
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