Ok, you’ve hired an investigator and have the proof, can you salvage the marriage? Maybe. Here is a link to a site with some insight into that.
The answer is not always to hire a lawyer. Is the cheating terminal, it depends of you, your spouse and all the details of your lives together. Perhaps your church has different rules for divorce than the state does? Are there kids, will the kids be better off if you stay together or if you split? Are you able to move forward in the marriage knowing the truth? Will you and your spouse be able to live with the consequences of the infidelity and move forward in the marriage if the truth is squarely on the table? Has all the truth come out?
Many times finding the truth is a small part. But, can you deal with the situation at all without knowing the truth?