What's there to explain? When you're a Communist, you follow a charismatic leader, you obey the rules and regulations of the ruling party in a dogmatic manner, you believe in a bright and glorious future coming any day now, and you are 100% sure your way is correct and all the others are wrong.
Never heard that quote, but "strength" is not "good". For instance Hitler and the Nazis were strong - so much that it took the cooperation of many nations to defeat them. Doesn't make them good or admirable.
Communism sees organized religion as a tool to manipulate the masses. Religion inspires fanaticism and that blinded collective momentum is easily used by the ruling class to further whatever political agenda they have.
Communism as implemented by Stalin was very much like a religion and it used strong emotions to support it. Patriotism, in general, is in many ways very similar to religious beliefs and passion .
Firstly note that a 'strength' is not necessarily a good thing. It can just be an observation that Einstein was making.
It sounds like Einstein was trying to explain that Communism works on a similar premise to many religions, without room for independent thinkers.
though combating against the religion, the communism gladly uses the most religious mind manipulations for its own gain.
saints replaced by the hero workers and hero soldiers
divine beings replaced with the founders of the Party and recent leaders
maneuvering masses through simple narrations and promises of better future