Don’t forget that virtually all of Russian Jewry trapped in the post WWI Soviet Union were lost religiously. Their is an interesting theory as to how the Communists were able to virtually wipe out religious observance in one generation while the Czars couldn’t accomplish it in two hundred years.
The Czars had tried persecution and oppression, which by itself never works. The communists, many of whom were Jewish and grew up in religious homes, knew better. They knew that if they cut off the figurative head, the body would die. They set to close shuls and yeshivas and kill/ exile all leaders end educators ( rabbanim, melameds, butcherd etc. With them gone, the community withered