I think this comes about due to the fact that in the US there is a group defined as the "religious right". The simplistic opposite view is the "secular left" despite the religious / secular spectrum only being loosely related to the spectrum of political ideology. There is definitely a correlation, but it shouldn't be used as a basis of assumption regarding religiosity or political ideology.

For example, Jesus Christ would be a good example of a religious left. He advocated taxes, social programs for the poor, and the moral ill of wealth. His view was less populist because he felt that this change should happen by those with power and money, but still very left even by modern standards. If Jesus' economic philosophy was made law, it would probably be the most socialist society in the world.