Can a Christian file a countersuit against someone who threatens their business for social reasons?

It does not seem to me that the baker has any basis for a suit.  Potential customers have the right not to patronize businesses for any reason, including that they do not like the business owner's position on political issues, regardless of the motivation for that political position, whether it be religious or otherwise.  Thus, for example, the bakery owner has the right not to patronize a business which provides benefits to same sex couples even though same sex marriage is not recognized in their particular state.  The bakery owner could even try to organize a boycott of that business.   People have the right to exercise their religious beliefs and the Constitution prohibits governmental sanctions against that religious practice (except where it conflicts with law, such as where someone practices a religion which believes in human sacrifice, or the use of illegal hallucinogenic drugs), but the Constitution does not prohibit individuals from "voting with their pocketbooks" when they disagree.  Thus, for example, a business may decide not to open on Sunday for religious reasons, but it has to acceept that it may lose business, or even be forced out of business, if all of its competitors offer convenient Sunday hours.