Bush is now the first former U.S. President to reach that age.
Bush family spokesman Jim McGrath said called the birthday a "very low-key day with family". "My faith has never been stronger", the former president wrote in the letter.
Bush has been hospitalized twice since Barbara Bush's death and several times in recent years.
Indeed, the family gathered alongside Bush in ME as he celebrated a week long's worth of birthday festivities, including what would have been his dear wife Barbara's 93birthday.
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As president, Bush encouraged others to be "points of light", reflecting his belief that people need to help out in their communities. Bush is celebrating his 94th birthday in Maine. He might not be the only former president to hit that mark - former President Jimmy Carter turns 94 on October 1.
He became the first-ever former US president to reach the age of 94.
The Central Intelligence Agency also marked the birthday by releasing declassified material related to Bush's tenure as director of the agency.