How does the US Supreme Court keep the justices from using their religious background to influence their decisions?

Answer 1

It doesn't. Justices make decisions based on their personal beliefs regarding proper constitutional interpretation, but those beliefs are always informed - as they are for the rest of us - by life experience, including religious background, education, individual psychology, social mores, and other factors.

No 21st-century US Supreme Court justice would use religion as a direct argument for or against a position in any case involving government policy because he or she would be aware (and the other justices quick to point out) that the First Amendment Establishment Clause prohibits the government for taking, or refusing to take, action on religious grounds. While this doesn't prevent such beliefs from influencing individual votes, it does ensure that they are well rationalized according to constitutional principles and buried in more neutral rhetoric.

For example, Justice X may believe abortion is immoral, consider it murder, and wish to overturn laws allowing this medical procedure. He or she would never present immorality as an argument against abortion; instead, the justice would be more likely to focus on the "State's legitimate interest in preserving life," or that a federal law supporting abortion unconstitutionally deprives states of the right to regulate medical decisions, which is not a power reserved to the federal government. Or the justice may argue against the right to privacy being a fundamental liberty in all cases, concluding that certain circumstances may create exceptions to protections allowed under the doctrine of substantive due process. Or the justice may use medical evidence to justify arguing to reduce the age of viability, taking an incremental approach to eliminating abortion.

No one reading an opinion couched in those terms would be able to prove religion was an influential factor, although people may suspect it was, and may even be correct.

On the other hand, Justice Y may hold similar personal beliefs to Justice X, but may also strongly believe in the right to privacy or that certain medical decisions, or certain stages of medical decision, fall outside the legitimate scope of government interest, and may support abortion legally without supporting it personally.

It's important to remember that all decisions are made by a simple majority of votes. No single justice controls the outcome of any decision, and must contend with the beliefs and persuasive legal arguments of other members of the Court.

There is no way of establishing as fact what personal and group dynamics influence the outcome of a vote, and no way of knowing any justice's private thoughts on matters before the court, or how much weight the justice gave to various (and sometimes conflicting) personal beliefs; therefore, there is no practical means of preventing someone's religious background from influencing his or her decisions.

For more information, see Related Questions, below.