Mickeyfan2,
From what I read, it's more than an agreement. The US Supreme Court first ruled, when it was challanged by members of the Amish and others, that the initial compulsary nature of the SS program participation was unconstitutional. Congress then passed additional legislation to ammend the program to add an religious objection exemption to the SS program. But like you noted, it's a total "opt out" and you are waiving the rights to any SS benefits as a condition of not having to pay into the program. Such exemptions are extended to Medicaid/Medicare withholdings as well.