A Songs of Praise special next Sunday will make the claim as it explores the inspiration behind her charity work.
The BBC's flagship religious programme will include interviews from friends that will say she had "stronger faith than people give her credit for", according to The Sunday Telegraph.
Leading presenter Aled Jones will also recall being invited to sing for her personally in her Kensington Palace living room, while the congregation of All Saints Church, Northamptonshire - near her Althorp family home - sing favourite hymns.
Mike Whitlam, former director general of the British Red Cross who worked with Princess Diana on charity campaigns including her landmine trip to Bosnia, also makes contributions to the programme.
While he was asked to speak at many outlets to mark the princess' 20th anniversary since her death, he said he agreed to appear on Songs of Praise because of its religious focus.
He added that he believed that a form of religious conviction helped her to persevere with her charity work.
He said: "She wanted to put love where there was hatred, and make a huge difference to people's lives so that they could live a better life.
"When you talk to people about making the world a better place, there are not many people who think it's doable. She did."
Tracy Borman, joint chief curator of Historic Royal Palaces, in an interview said Princess Diana's compassion and charity work "of course absolutely reflects very Christian values too".
The programme is one of many documentaries about the princess or involving her sons.
The Channel 4 documentary Diana: In Her Own Words, divided opinion among commentators with reviews ranging from "trashy" to "heartbreaking".
The Songs of Praise special will focus closely on Diana's charity work, alongside hymns including I Vow to Thee My Country, which was played at her wedding and funeral.
