According to tradition, water from the River Jordan is set to be used for Louis's christening.
Queen Elizabeth, 92, has also begun to scale back her duties because of her advancing age, handing over some of her responsibilities to her heir, Prince Charles.
Her Majesty's decision not to appear at the christening was not based on health grounds and was reportedly agreed with the Cambridges a while ago.
Louis is fifth in the line of succession to the British throne after his grandfather Prince Charles, his father William and his older siblings George and Charlotte.
The Duke and Duchess of Cambridge have chosen close family friends including Guy Pelly and Nicholas van Cutsem as godparents for their third child, Prince Louis.
Archbishop of Canterbury Justin Welby, the spiritual leader of the world's Anglicans, lead the service in the Chapel Royal at St James's Palace.
Kate replied that the baby looked "very relaxed and peaceful" and said: "I hope he stays like this".
There were 30,000 copies in the first run of AA Milne's classic tale of the lovable bear and his friends in Ashdown Forest, East Sussex.
The robe, which was handmade by Angela Kelly, the Queen's personal dressmaker, is a replica of the original christening gown designed for Queen Victoria's eldest daughter.
This is the first time the family of five has been seen as a full unit in public, People reported.
(Charlotte's ceremony took place at St. Mary Magdalene Church, in Sandringham, instead.) Kate Middleton herself also received communion at the Chapel Royal just before her wedding to Prince William.
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