What is Syria's primary religion?

It depends on the time period. Do you mean ancient Syria or modern Syria?

With regard to ancient Syria, the Syrians were Semites, closely related to and associated with the Israelites. Yet in the eighth century B.C.E. there was sufficient difference between their languages that the common Jew did not understand Aramaic. (2Ki 18:26-28; Isa 36:11, 12) Also religiously, there were vast differences between the polytheistic Syrians and the Jews, and it was only when the latter apostatized that worship of the Syrian gods was allowed in the land of Israel.—Jg 10:6; 2Ki 16:10-16; 2Ch 28:22, 23.

Modern day Syria is a mix of Islam and Christianity.