It is the tension between sects of the same religion that has more tension. Different religions have different beliefs. It's easy to understand their conflict.
The reason why there is tension in the same religion is because one believes the other puts shame in their religion, or maybe, the other is accused of not doing the right attitude of worship to their deity.
I'm a christian who believes that our Lord Jesus Christ is the only begotten son of God. Now, imagine how hurtful it would be for me for someone who claims to be a christian yet he criticizes the deity of Christ.
To me, no tension surpasses that.