Two more relevant refs from wikipedia ~~

Trust in God and keep your powder dry is a maxim attributed to Oliver Cromwell, but which first appeared in 1834 in the poem "Oliver's Advice" by William Blacker with the words "Put your trust in God, my boys, and keep your powder dry!" The poem is a dramatic representation of Cromwell addressing his army during the invasion of Ireland. Edward Hayes, who edited the anthology in which the work first appeared, calls it a "well-authenticated anecdote of Cromwell".

According to the Pittsburgh Post-Gazette article of January 1, 1943, the origin of the quote may well have been John Ford's 1939 film 'Drums along the Mohawk'.