The book "Treatise on the coexistence of the faithful with the unfaithful. Appendix: a sermon on the recognition of Christ and His Gospel and on the rejection of it" is a joint work of two prominent figures of the Reformation - Pietro Martire Vermilli and Heinrich Büllinger. In this book, they consider the complex topics of the coexistence of the faithful and the unfaithful, and also remind us of the importance of open faith in Christ and His Gospel. All this is offered to the reader through the prism of their unique religious and philosophical approach, which makes the book significant for further research and reflection on spiritual issues.