Whether "certain" means dogmatically insistent on, logically convinced by, or temperamentally predisposed towards a given conclusion about god, freedom, or morality (etc.), acting as if only works in the pragmatic sense when belief and action are in accord, not when they contradict one another. And acting on beliefs rooted in one's temperamant and sensibility but inconclusively supported by coercive evidence is not dishonest, unless the evidence for a competing conclusion is compelling.
Consider James's climber:
Suppose, for example, that I am climbing in the Alps, and have had the ill-luck to work myself into a position from which the only escape is by a terrible leap. Being without similar experience, I have no evidence of my ability to perform it successfully; but hope and confidence in myself make me sure I shall not miss my aim, and nerve my feet to execute what without those subjective emotions would perhaps have been impossible. But suppose that, on the contrary, the emotions of fear and mistrust preponderate; or suppose that, having just read the Ethics of Belief, I feel it would be sinful to act upon an assumption unverified by previous experience,--why, then I shall hesitate so long that at last, exhausted and trembling, and launching myself in a moment of despair, I miss my foothold and roll into the abyss. In this case (and it is one of an immense class) the part of wisdom clearly is to believe what one desires; for the belief is one of the indispensable preliminary conditions of the realization of its object. _There are then cases where faith creates its own verification_. Believe, and you shall be right, for you shall save yourself; doubt, and you shall again be right, for you shall perish. The only difference is that to believe is greatly to your advantage.The mountain abyss is no impenetrable brick wall, or at least the climber is not compelled to think so. It is a daunting challenge, which our climber will meet only on condition that he muster his most vital "subjective energies." Unless he's a fatalist with a death wish, he'll regard the outcome of his plight as indeterminate, but possibly responsive to his best effort. Sometimes, as Icelanders know, "being stuck is a state of mind."
Irwin's final paragraph acknowledges the disingenuity of asserting religious and ethical "fictions" and violating one's own convictions, but treats free will as a special case. "I cannot believe in free will, but I can accept it." No. When you face your terminal leap - as we all do, much more frequently than we know - you'll be a believer.
And that's a nice (unpremeditated) segue to Kierkegaard on Monday... though possibly not to Russell on Tuesday.