The more you resist rather than accept the Midlife Crisis process, including what you or your spouse define as sin, the more he will fight against you, distracting himself from his personal issues within his crisis. Resistance takes the form of accusations of sinning in general and specific accusations such as infidelity, often in partnership with threats of divine punishment or damnation. Grace, Agapé and Forgiveness are more effective than judgment, but even they will not stop a Midlife Crisis that is in progress.
Regardless of previous, present or future beliefs about marriage and monogamy, a Midlife Crisis Spouse will likely act against those beliefs. This brings us to the question: What is sin? Sin has many definitions and interpretations.
Those ideas contain quite a few subjective terms. What is immoral--who defines immorality? Since it varies across culture and religion, not everyone agrees on what constitutes right and wrong and since these terms are not absolute, they can only be understood within a certain context; outside of a specific context, there is no right and wrong.
If sin is a turning away from God, the moral violation is based on the individual's personal set of beliefs--what that person believes to be immoral, wrong, shameful, harmful... Each person has different beliefs--though they may be in alignment with the beliefs of others. Beliefs differ not merely across broad religions such as Christianity and Hinduism, but even within a single church community. A sermon given to a group will have as many interpretations as there are listeners. Thus sin is a turning away from a person's beliefs or values in relationship with God, Self and others--the world. Beliefs change, and a life transition is a place of paradigm shifts--a transition is the process of changing from one form, or stage to another. But crisis-level change is often not about change so much as it is about rebellion and fear of change from beliefs still felt in one's inner Self; it is thus a turning away rather than a healthy transition.
MLCers may seem happy and certain with their new life changes. How can you tell that a person's transition is a crisis-level rebellion rather than a healthy shifting of one's beliefs? Guilt. Crisis and non-crisis transitions may include confusion and mixed emotions, but there is a greater level of shame and toxic guilt when it is a rebellion. But not all MLCers will reveal their guilt externally. Instead you may experience projection and blame from an MLCer who denies or fears their guilt.
Accept the guilt, anger, betrayal...as part of the process of crisis while understanding that accepting is not an endorsement. It is a Surrender, an admitting that these turmoils are not yours to control or eliminate from your MLCer's experience.