Speaking on Premier's News Hour, Catherine Butcher co-author of 'The Servant Queen and the King she serves' with Mark Greene, said "she's someone who is very clear in speaking about her personal faith but not in a way that excludes others. I've always thought of her as Britain's best female evangelist because she does talk about Jesus."
Gun salutes have rung out across the UK to mark the Queen becoming the first British monarch to reach their Sapphire Jubilee.
The band of the Household Cavalry outside Buckingham Palace
Members of C Troop 211 Battery fire a 21-gun salute in the grounds of Cardiff Castle
Catherine Butcher said that the day is one of sadness for the Queen. She told Premier: "It is the anniversary of her father's death and so for her it's a very sad day. She's reached this momentous milestone only because her dad died young at 56."
The Queen acceded to the throne following the death of her father George VI on 6 February 1952.
She spent the day privately at her Sandringham Estate in Norfolk.
The Archbishop of Westminster Cardinal Vincent Nichols was among the well-wishers on Twitter. He wrote:
Congratulations, Your Majesty, on your #SapphireJubilee. Your faith and commitment are an inspiration to all. https://t.co/aBIeEjMEe1
— Cardinal Nichols (@CardinalNichols) February 6, 2017
Listen to Aaron James speaking to Catherine Butcher here: