Tā shì yī gè jiāndìng de gòngchǎnzhǔyìzhě, xìnfèng Mǎkèsīzhǔyì.
He is a committed communist who believes in Marxism.
Tips
usage
共产主义者 refers to someone who holds communist beliefs as an ideology. This is distinct from 共产党员 (a Communist Party member), which is a formal organisational membership. One can be a 共产主义者 without being a 共产党员 and vice versa.