According to reports, Harry marked the occasion with an incredibly rare first-edition copy of A.A. Milne's Winnie-the-Pooh - dating back to 1926 - which he bought from Peter Harrington, a rare book store in London.
The Archbishop of Canterbury Justin Welby welcomed the royals at the entrance to the chapel.
The royal couple's youngest son was christened at the Chapel Royal at St. James's Palace - the same church where his brother Prince George was christened.
Kate's parents, Carole and Michael Middleton, were also in attendance, alongside her brother James and pregnant sister Pippa, as well as Pippa's husband James Matthews.
It was a big day for Prince Louis. Then again, after such a generous gift maybe William and Kate will change their minds?
"'Robinson Crusoe' was William's favorite book, but Harry loved all things A.A. Milne". He is fifth in line to the throne.
Although the adults and the air performance were awesome, it was Princess Charlotte and Prince George who truly stole the show.
Prince Louis has three godfathers and three godmothers altogether: Nicholas van Cutsem, Guy Pelly, Harry Aubrey-Fletcher, The Lady Laura Meade, Mrs Robert Carter and Miss Lucy Middleton.
The little prince wore the same gown his older siblings wore to their christenings and continued the tradition.
During his christening, Prince Louis was wearing the handmade replica of the Royal Christening Robe, which in 2008 replaced the original which had been worn for all royal christenings since 1841 when it was first worn by Queen Victoria's eldest daughter.
Following the service, where Prince Louis was flanked by six godparents, the royals made their way to Clarence House where they enjoyed an intimate afternoon tea with guests.