It's been several years, and someone had tried explaining it before (I can't find it), but can someone be helpful enough to explain why some things in the old testament are to be lived by and some things aren't? I vaguely remember something about it being because Jesus fulfilled one thing but not another, something along those lines. Like I said, it's been several years.

I'm asking because every day the last few months, ever day, I am reading comments from everyone from politicians on down to know-nothing-teeny-boppers saying things like "well, if you are so against the gay why are you eating shellfish or wearing blended/mixed garments." sometimes worse and verbose, usually mundane and repetitive. I just would like a reminder and/or clarification on it because even though I freely admit to some of the ones I talk to I'm no theologian scholar, they insist on trying to force me to explain something I know nothing about.

I hope I'm clear on the request.