Citizens must be free to live their own lives without fear of those who would legislated their freedom based on myths and old stories for myths and old stories sake. Religion has separated minds and civilizations for thousands of years. It's ridiculous and unacceptable behavior in a modern civilized world. Moving forward will require we separate ourselves from archaic thinking and work together.
Being a strong believer of the idea "History repeats itself" I can support the idea that this combination could leave to tyranny. The practice of one single religion will create a cult-like government and diminish free expression and suppress free thinking. People should be supported to express their own beliefs fully and express support towards others to do the same.
Look at how things have played out throughout human history. Over and over again, religion and politics do not mix. The thing about it is that, normally, religion and government combining can be okay. In the light of keeping a society cohesive and unified, I have no doubt that it can even be a positive thing.
When things get bad, though, religion and government mixing almost invariably leads to tyranny. Look at how history has played out over and over. There will always be bad times just around the corner, and there is no avoiding this. So, yes, the combination of religion and government leads to tyranny.
Religion and government should not mix. Religion tells people that absolutes exist and that God is the most important thing. This means that the government does not have the well-being of its people as its number one priority. By definition, they would have to have the religion or the wishes of God as their number one priority. This leads to the government quashing any opposing points of view and thus leads to tyranny.
You typically don't see tyranny in governments with Christian leaders, though that has not always been the case. History, of course, is somewhat subjective. Plenty of historians would argue that the leadership of King Henry VIII of England, for example, was somewhat tyrannical, and he flip-flopped between Christian-based religions during his reign. However, it is difficult to see the current turmoil in various Middle Eastern countries and not pin at least some of the blame on an unhealthy mix of religion and politics.