Res publica christiana is a Latin phrase combining the idea of res publica and christiana to describe the worldwide community of Christianity and its well-being. A single English word with somewhat comparable meaning is "Christendom".
This phrase is notably found in the encyclical Christianae Republicae Salus, a Roman Catholic Church document which condemns Voltairian and masonic indifferentism in matters of religion.
References
Res publica christiana Wikipedia(Text) CC BY-SA