Using the bible as a reason?

Michelle • 21 year old FTM due January 2016 ❤️
If you are Christian or practice another religion, can you explain why people pick and choose certain things to follow in the bible (or other religious document) and ignore others? I am not religious and this has always confused me.
My question: do you choose what you want to follow and what you ignore? And how do you pick and choose what to ignore and not follow? 
Ie: will choose circumsion for religious reasons (the bible says so) but don't follow the belief to own slaves anymore... 
Genesis chapter 17, verse 12:
And he that is eight days old shall be circumcised among you, every man child in your generations, he that is born in the house, or bought with money of any stranger, which is not of thy seed. He that is born in thy house, and he that is bought with thy money, must needs be circumcised.
In this passage God understands that people buy other people and, quite obviously, is comfortable with the concept. God wants slaves circumcised in the same way as non-slaves.
Thanks! 

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