first of all, church attendance alone is not a credible indicator nor the sole criterion of overall religiosity; it involves Poland's history, culture, lifestyles and overall heritage. Secondly a once-a-year attendance check in autumn is also dependent on external factors including the weather and rival events on that paritcular day.
There are many ways in which one can measure the level of crime in Poland: raw number of offences, number of offences per hundred thousand people, chance of being a victim of crime, chance of being affected by crime, chance of being a criminal, etc. And there are various ways in which one can measure the level of religiousness in Poland: number of people who meet the minimum criteria required by the Catholic church, number of people who take communion every week, number of people not getting their kids baptised, number of people leaving the church, etc.
What they all have in common is that almost every one of them is showing that crime is going down and so is religiousness. If your hypothesis that Poland had low crime because it was a religious country were to be correct, we would expect a fall in crime to be accompanied by an increase in religiousness, and we would expect a fall in religiousness to be accompanied by an increase in crime. But the reality is that for pretty much all of this century we have actually seen falls in both crime and in religiousness.