'Res publica christiana' is a Latin

Latin

Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 phrase combining the idea of res publica

Res publica

Res publica is a Latin phrase, loosely meaning "public affair". It is the root of the word republic, and the word commonwealth has traditionally been used as a synonym for it; however translations vary widely according to the context...

+ christiana to describe the worldwide community of Christianity

Christianity

Christianity is a monotheistic religion based on the life and teachings of Jesus as presented in canonical gospels and other New Testament writings...

 and its well-being. A single English word with somewhat comparable meaning is "Christendom

Christendom

Christendom, or the Christian world, has several meanings. In a cultural sense it refers to the worldwide community of Christians, adherents of Christianity...

".

This phrase is notably found in the encyclical Christianae Republicae Salus, a Church document which condemns voltairian and masonic indifferentism in matters of religion.