This paper argues that it is not reasonable for a critical rationalist to be a religious believer in the Abrahamic tradition. The argument is distinctive, in that it takes seriously the critical rationalist view that we should abandon ‘justificationist’ argument. What this means, is that the structure of argument then becomes a matter of offering theories as resolutions of problems, and then judging how they fare in the face of ongoing critical appraisal. The paper surveys issues in several areas, including God and what is good; homosexuality; mysticism; and messianic ideas. It argues that, unless the believer engages in intellectual retreat (something that is unacceptable for the critical rationalist), the problem-situation facing the believer appears to be so daunting that it would be unreasonable for them to hope that they can overcome it.