son cristianos

are Christians

→ ser
phrase Rare #37,164
are Christians Used to describe people who follow Christianity. "Son" is the plural form of "to be" here.
SP: Mis vecinos son cristianos.
EN: My neighbors are Christians.

Ellos son cristianos.

They are Christians.

Pero ustedes son cristianos.

But you are Christians.

— Incendies (2010)

A Tom le gustan los ucranianos porque son cristianos.

Tom likes Ukrainians because they are Christians.

A Mary le gustan los italianos porque son cristianos católicos.

Mary likes Italians because they are Catholic Christians.

Son cristianos.

They are Christians.