Q: My husband had an affair. We have gone to counseling and are trying to work things out, but how can I really trust he will never do it again?
A: First of all, I applaud you for your efforts to restore your marriage in the aftermath of your husband's infidelity. A breach of trust that deep is difficult to recover from, and many couples are not willing to do the rebuilding effort.
To answer your question, you cannot know for certain he will never be unfaithful again. Choosing to love another person always involves an act of faith, hoping for what we cannot be certain about. However, no marriage can be based on "blind faith." In a marriage relationship, you and your husband owe it to each other to demonstrate a commitment to fidelity. This is particularly true as he has been unfaithful in the past.
Working with your counselor, you and your husband need to build safeguards or "hedges" around your marriage to protect against another affair. For example, do you have access to each other's cell phones and email accounts? Obviously, you could go overboard checking up on your husband, feeding an atmosphere of distrust and even paranoia. But in the wake of an affair, it is reasonable for you to expect a greater level of accountability in order to rebuild trust.
Another critical element of preventing another affair is understanding how the first one occurred. In many cases, affairs happen because there are cracks in the marriage. Perhaps you and your spouse drifted apart, stopped communicating, and left each other lonely in the process. Maybe there were unresolved issues related to finances, sex, or parenting. Sometimes, an individual may have emotional problems such as past sexual abuse or bipolar disorder that lead to an increased likelihood of infidelity. Work with your counselor to identify what made your marriage open to the affair initially, then come up with practical ways to strengthen those weak areas.