Christ dieing on cross and god's criteria?

hello, well i don't know a lot about religion, anyway if christ died on the cross for our sins, wouldn't that give us a 'free ride' and what is god's criteria for sending someone to hell? if you tell a 'white' lie, he's not going to send you to hell but if you are a serial killer, well that would be a different story. so how does he decide heaven or hell? thanks.