ELI5: If you're a Christian, why do you have to be good if Jesus will forgive you no matter what?

I grew up going to church but I'm not religious now and I never really understood this part.

Please, no answers along the lines of "aha, that's why Christianity is a sham" or "religions aren't logical". I don't want to debate whether it's right or wrong, I just want to understand the logic and reasoning that Christians use to explain this.