This is an exaggeration, religion for Marx is a very ambiguous force. Like opium, it is both a poison AND a painkiller. It crushes revolutionary energy AND is necessary for people to survive the day to day violence of capital. https://twitter.com/beka_valentine/status/1357206430114148352

Overall, Marx rejected a monolithic view on 'religion' - he saw it as an institution of capital, penetrated by the very same historical contradictions as capitalism.

And just like capitalism the inner contradictions would eventually give rise to the institutional successor of religion.

He says :
"To develop in greater spiritual freedom, a people must break their bondage to their bodily needs [..]. They must, above all, have time at their disposal for spiritual creative activity and spiritual enjoyment."

And you see that while spirituality is seen as essential, the Church is conspicuously absent here !

TL;DR :

Spirituality is essential to freedom.
Organized religion promotes spirituality but means to put it on a leash.
Therefore religion is both a tool of control *and* a promoter of liberation. It holds the revolution back *and* participates to its inevitability.

Also to note : Post Marx marxism was deeply impacted by the specifics of religion where it was happening, as it should.
For example in Russia, the orthodox church was a powerful feudal entity on par with the Tsar in land ownership. That was a key element of bolshevik atheism !