It’s not really necessary to know your heretics, unless they’re doorknocking you.
Even then you can find ways to avoid them or annoy them. I keep a Greek New Testament on hand. When they deny that Jesus is God, I mention John 1:1. “In the beginning was the Word, and the Word was with God, and the Word was God”. I point out that in verse 14 of the same chapter, the Word becomes flesh—God becomes man, the man Jesus Christ.
When they scoff at the unreliability of modern translations of the Greek, and show me their special approved version which tells me that no, the word was not God but ‘a god’, I excuse myself and find my Greek New Testament. I confess that my knowledge of Greek grammar is, unlike my sinful soul, beyond redemption, and ask them to help me out by explaining themselves further. We carry on from there. It is an enormous amount of fun of the 2 John 10 variety, assuming you have a lazy half hour on your hands. And you are fulfilling the command to love your neighbour, because you are allowing them an extra half hour of sleeping in by detaining those who would disturb them (these heretics always seem to doorknock our house on Saturday mornings).
But while the Bible doesn’t particularly encourage us to know our heretics (see the aforementioned 2 John 10), at times it can be quite useful to be able to recognize them. So if you need a simple, useful spotter’s guide, check this series.
Here’s a good example on the subject of monothelitism, in which Justin Holcomb, the author of the series, explains how ‘Jesus is controlled by skynet’ is not a good or right way of looking at life.
If you find yourself needing to recognize a heresy, it’s recommended! (With thanks to Justin Taylor.)