Both religion and politics are concerned with social organization, ethics and the moral standards that affect them, so their areas of interest are largely the same. Sometimes religion will monopolize one or the other of these areas of interest, sometimes government will.
In early Jewish or Islamic societies "government," and "religion" were essentially synonymous.
A thousand years ago the Roman church largely monopolized morals, ethics and social organization in Western Europe. Today in the West social organization is largely the purview of a secular government, morality of religion, and ethical conduct a shared concern.
As long as religion and secular government share the same areas of concern, there will be a certain degree of competition.