You're asking loaded questions; ones that have been asked for centuries...books and books have been written on them, denominations have come about as a result of them...these are not trivial questions that are easily answered with one liners...maybe that's why people aren't answering your question.<br><br>
My (shortened version for this purpose) answers to this have revolved around the "Old Covenant" and "New Covenant" view of the Bible and of time. God originally chose the Jewish nation as HIS people. He gave them rules to follow so that they would receive blessing upon blessing. These rules were intended for their purity and perfection and benefit. And also to show that there is not one person, nor has there been one person (other than Jesus) to live a perfect life. No one has followed all of the 10 commandments (at the very LEAST) for their entire life. No one is blameless.<br><br>
In order for an unholy people to be in the presence of an holy God, restitution must be paid...they must be made holy. Back in the Old Covenant, the way to holiness was via sacrifice, ritual, tradition, following the rules.<br><br>
I'm not sure why or how or when or whatever...but God changed it up. Maybe He got greedy and wanted everyone, but He decided on a New Covenant. The one where anyone is free to join and all sacrifice has already been paid by the ultimate sacrifice of the one perfect One - Jesus. And this covenant rests, not on works, but on grace. Jesus paid your debt for you. You are no longer remanded without bail, you have surety who has paid for you and you are free to go. NOW, as a result of that huge gift, your heart changes and you aspire to a life of purity...of following the greatest law of all: "Love the Lord your God and your neighbor as yourself." It's a change in perspective. You aren't working to receive salvation anymore. Salvation has been granted to you when you take Jesus as your savior. As a result of that taking, your actions change because you have changed.<br><br>
How can you know what you've done wrong unless you know the rules you've broken? The OT gives us that list...that shows us in every single way we've "missed the mark." The NT gives us the hope that missing the mark isn't a death sentence.<br><br>
And there is so much more to be said. And other questions will emerge for which I may or may not have time...but...the difference between the old and new testaments is huge and small all at once.