Attributed by William Blacker (not to be confused with Valentine Blacker), who popularized the quote with his poem "Oliver's Advice" http://books.google.com/books?id=JmEaAQAAIAAJ&dq=%22Oliver%27s+Advice%22+Cromwell&q=%22Oliver%27s+Advice%22+Cromwell#v=snippet&q=%22Oliver's%20Advice%22%20Cromwell&f=false, published under the pseudonym Fitz Stewart in The Dublin University Magazine, December 1834, p. 700; where the attribution to Cromwell appears in a footnote describing a "well-authenticated anecdote" that explains the poem's title. The repeated line in Blacker's poem is "Put your trust in God, my boys, but keep your powder dry".
Attributed
Varianta: Trust in God and keep your powder dry.
Varianta: Put your trust in God, but keep your powder dry.