What was Alexander Graham Bell's religion?

Quoted from ''Alexander Graham Bell and the Conquest of Solitude" The official Bell biography by Robert W. Bruce. Pg 490. "In 1901 Bell came across a Unitarian pamphlet and found its theology congenially undogmatic. 'I have always considered myself as an Agnostic,' he wrote [his wife] Mabel, 'but I have now discovered that I am a Unitarian Agnostic.' "