Is it unlawful to deal with Jews and Christians in business?

There is no objection to a Muslim working in a company if their work is Islamically permissible. There is also no objection to dealing with non-Muslims in buying, selling, leasing, and the like, provided that the buying and selling are in matters that the Sharia has permitted. However, selling prohibited items such as alcohol, pork, or crosses, or leasing places to them for use as churches, is not permissible.