Why do I fight against Calvinism?

A lot of Christians believe it to be a waste of time and energy to contend for certain Christian doctrines and/or to oppose other Christian doctrines. 

Shouldn't Christians simply try to unite over and against the non-Christian ideological and non-Christian religious forces of the world? 

For many doctrines, I would agree wholeheartedly. I don't think Christians should be fighting each other with regard to Baptism, to Eschatology, to Church Government, to Communion, to the Gifts of the Holy Spirit, etc. etc. I'm not saying that there aren't correct views and incorrect ones with regard to all of the aforementioned doctrines, but none of them say anything essential about God's character and/or His essence. 

In short, there are lots of doctrines we can agree to disagree upon. Especially the doctrines that don't have anything to do with maligning God's character. Take for example the issue of the mode of baptism. Whether we sprinkle someone or immerse them entirely doesn't say anything about God's character. I'm not saying one isn't more Biblical than the other, but it is truly a secondary issue. 

But, when someone teaches/asserts that God doesn't love everyone (as Calvinism does), that God hasn't provided for everyone's salvation (as Calvinism does), that God is the author of sin (as Calvinism does in practice, although they deny it vehemently), that God created people for hell (as Calvinism does) - these are NOT doctrines of secondary importance. These are doctrines that MISREPRESENT the God of the Bible, that reveal God to be someone He is not. Beyond that, we know that such doctrines were not the Apostolic faith that was handed down and preached/taught by the early Church, rather they are accretions that were introduced in the 5th Century by Augustine. And for these reasons, I oppose the doctrines of Calvinism with energy and unapologetically.

They are not only wrong, but they are in fact FALSE. They contradict the original, the Apostolic, Christian faith. They originate in the teachings of pagan philosophy and in Gnostic heresies. They should therefore be opposed, refuted and taught against by faithful Christian teachers.