The central causes belli was the continuing Russian expansion to Balkans and overall effort to dismember the decaying Ottoman empire with an ultimate view of influencing in certain way the rights of passage through Dardanelles... or even controlling the Constantinople itself. Basically the religious issues were a sort of public cover story... Western powers tried to protect the Ottomans, because Russian empire would expand too much in case of successful war and a victory would encourage the tzars to engage in even more ambitious projects (Afghanistan for example).

Whilst there's always more to be said. Here's an answer by /u/lordtsubodai and an answer here by /u/banal_penetration

These are older answers and may not meet the standards of today.