Belief and faith
The gulls are "gullible," easily led to lend their belief to the tricks and plots of the conmen. The play itself is obsessed from the Prologue onward with the idea of what Coleridge would call the "willing suspension of disbelief," except that the gulls do not really start with much or any disbelief, and this is reality for them, not a story in which they believe the premises of a story in order to see what the author does with it. As Jonson's audience, we know that the stories (and the whole play) are not real, so we are not gulled.Jonson, in portraying two Christian believers, explicitly considers whether there is a difference between having faith in the particulars of a Christian denomination--or having faith in God, or in anything transcendent--and believing in the false tricks of the conmen.Belief of course is essential to theater, and the play's many metatheatrical forays play on this theme.