The baby wore a handmade replica of the Royal Christening Robe, which was made in 1841 for the christening of Queen Victoria's eldest daughter, according to Kensington Palace.
The Duke and Duchess of Cambridge chose two hymns for the ceremony - "O Jesus, I have Promised" and "Lord of All Hopefulness".
PA Wills and Kate leave with their kids Prince George Princess Charlotte and Prince Louis
The UK's Press Association said the decision is understood not to have been taken for health reasons and was mutually agreed by the Queen and Louis' parents, the Duke and Duchess of Cambridge, some time ago.
The christening was only Prince Louis' second public appearance. Fellow godmother the Lady Laura Meade is the wife of Prince William's longtime friend - and another Charlotte godfather - James Meade, while the third godmother, Mrs. Robert Carter (aka Hannah Gillingham) went to school with Kate and continues to be close to the duchess. The latest baby of Kate Middleton and Prince William is just adorable in this noble ornament, and the whole family was also very elegant as usual.
Prince William and Kate Middleton's wedding cake.
Kate's cousin Lucy Middleton has been named a godmother; her brother, Adam Middleton, now serves as one of Princess Charlotte's godfathers.
"She loved all the old classics and Harry had the brilliant idea of starting a little library of first editions for Louis, Charlotte and George to enjoy as they get older".
At the end of the service the duchess fanned herself with the order of service against the stifling heat as she walked from the chapel.
Kate is expected to spend the rest of the summer and possibly some of the autumn looking after her young family.
While all eyes were on Prince Louis during his royal christening, we couldn't help but notice the venue's sweet connection to Princess Diana.