Religion was very important to people at least around the time of the Roman Empire's collapse (4th century onwards). Ordinary people around the Mediterranean used to argue about the nature of God and the doctrine of Trinity while standing in line at a shop!

The ecumenical councils of the day, for example, were massively important political events - the Council of Nicaea certainly held no less significance for the people under their jurisdiction than, say, the Nuremberg trials did for contemporary Europeans; it issued a controversial yet generally accepted verdict on what's OK to believe and what's vile heresy. I'm making that comparison because Christianity used to occupy, among other spaces, the niche in public consciousness that nationalism took in the 19th century.