Why would you assume this? Didn’t all men in the ex-USSR have to serve in the military, or at least were under the obligation to serve if called? The militant atheist policy of the USSR tamped down religious belief among the various peoples of the Soviet Union, that is true, but they did not exterminate religion entirely. Millions of Soviet citizens were believers of various religions. Moving further west, in Poland, atheism was a non-starter — Party members were, in theory, not supposed to be religious, and some weren’t, but religious devotion wasn’t altogether absent among them. Both Yeltsin and Putin became publicly practicing Orthodox Christians after the fall of the USSR, and Yuri Gagarin was definitely a believer.One of my teachers at school a couple of years ago was a Ukrainian who spoke Russian and Ukrainian as his first languages.
He had been in the Soviet army so I’d have to assume he wasn’t religious.