Organized religion or the very concept of God itself doesn't exist in Marx's envisioned far-distant future utopia. "The opiate of the masses" was an ironic phrase meant by Marx and later socialist/communist idealogues for industrial proletarian workers to destroy, eliminate bourgeiouse, remnants of feudalism by smashing, closing or turning churches, synogagues even mosques into hospitals, hostels, or schools. If you were a believer, you'd be mocked, shunned, or discriminated against by being given menial, "go-nowhere" jobs with little socio-political, economic mobility.