Biblical Separation: The Struggle for a Pure Church by Ernest Pickering and published Classics.
This book is the classic historical and Biblical defence of an idea that the church cannot fellowship with those who deny the doctrines of the historic Christian faith. Biblical separation is the implementation of that scriptural teaching which demands repudiation of any conscious or continuing fellowship with those who deny the doctrines of the historic Christian faith, especially as such fellowship finds expression in organized ecclesiastical structures, and which results in the establishment and nurture of local congregations of believers which are free from contaminating alliances.
