According to the Instagram account Royal Letters, the letters were placed in an envelope with Kensington Palace's crest and included a black-and-white portrait of Markle and Harry from their wedding day.
However neither Prince Harry nor Meghan were named among Louis' six godparents, with the honour going to Guy Pelly, who was once dubbed the "court jester", family friend Nicholas van Cutsem and William's school pal Harry Aubrey-Fletcher.
While this is a meaningful day for Will and Kate's third child, July 9 already marks a memorable date for the Royal clan: It was on this day in 1947 that Queen Elizabeth and Prince Philip announced their engagement, and posed for photos at Buckingham Palace.
While William was also dressed in full RAF uniform, his ensemble differed slightly to Harry's by wearing a blue sash across his body.
Meghan looked sensational in a simple black dress with matching fascinator as she clung to her husband's arm.
The Duchess of Cambridge sported the special Air Cadets Dacre broach for the service, which sat on her Alexander McQueen coat dress, the same style she wore to Prince Harry and Meghan Markle's wedding. The dress was belted and featured a boat-neck neckline, a favourite silhouette of the Duchess's. She seems to be adoring American designers lately as the olive green quarter-sleeve dress is by Ralph Lauren. But it was largely hidden behind Prince Louis' christening gown. His step-grandmother, Camilla, and sister, Charlotte, also wore white.
One of the unofficial fashion rules is that Royal ladies can't wear all-black outfits unless it's for a somber or commemorative occasion.