Let's say someone has cited several psgs from Leviticus and suggested I pick and choose which to follow.
Not I, however, but the New Testament itself picks and chooses, and explains why.
Some parts of the Old Testament Law deal with
1) ceremonial/ritual purity,
2) punishments for breaking purity laws,
4) punishments for breaking moral laws,
5) civil laws, and
6) punishments for breaking civil laws.
Remember OT Israel was a theocracy, where God ruled the people directly and appointed human proxies; judges, high priests, and (reluctantly) kings later on.
1 & 2 - OT Israel had a state religion - the worship of the LORD. There are OT Laws that deal with the permissibility of approaching the sacrifices for priests, of cleanliness for priests and laypeople, of entering the temple, etc. One could become unclean by handling unclean food, going near a dead body, contracting a disease, etc. Ceremonial purity was necessary for several reasons:
-To set apart the LORD as absolutely holy in the minds of the worshipers.
-To set a standard of rigor for approaching Him; one must not do so flippantly.
-To set the Israelites apart from the surrounding pagan societies' religious practices.
-To foreshadow the coming of the Messiah.
Such laws are fulfilled in the coming of the Messiah, as the Epistle to the Hebrews goes to great pains to explain. My cleanliness and purity to approach God is based on Christ's perfect righteousness, granted to me by His grace on the basis of faith only, rather than rituals that I must perform. When I come to God now, God sees Christ's righteousness and perfection, not my impurity and uncleanness.
If we want to get technical, God would also see a true believer in OT Israel as clothed in Christ's righteousness. The difference is in the command for the actions of the community, and in the community itself. OT Israel was a community where everyone, believer and unbeliever alike, had the same obligations, whereas the New Testament church has different kinds of obligations b/c it has a different kind of membership. Salvation is the same (faith in Christ as Savior, not by anything we can do) but the context in which I live has been defined differently by God.
3 - Expands on the moral law that was already in place. This is an important point - the Mosaic Law was not the 1st occurrence of the moral law.
The moral code was partly innate and stated in forms here and there before the Mosaic Law. How did Adam and Eve know they weren't supposed to eat from the tree? God told them. How did Cain know he wasn't supposed to murder Abel? Perhaps God told him, perhaps it was an innate knowledge – in either case, he knew. How did Noah know the rest of humanity was "wicked" before the Mosaic Law? How did Abraham know that he should honor his guests w/ food and hospitality? How did Lot know that homosexual assault on his guests was wicked? That law comes from God and has been made known (mysteriously) by God to humanity.
Thus, just as the moral law did not originate with the Mosaic Law, neither does it come to an end when the Mosaic Law is fulfilled in Christ. One could think of moral laws as those which reflect the pre-Mosaic Law-law. They are thus still in force.
4 - That is not to say that the punishments for breaking the moral law remain the same.
A few examples:
Genesis 9:6 - Whoever sheds man's blood, By man his blood shall be shed, For in the image of God He made man.
Here capital punishment for murder is laid down in the earliest days of human existence. The Mosaic Law continues that punishment. Romans 13 refers to "the sword" to punish evildoers that the state is justified in wielding.
On the other hand...
No explicit condemnation exists until the Mosaic Law (though it was obviously known to be wicked before), and the punishment is execution.
But the New Testament gives no provision for the execution of homosexual offenders. Rather Romans 1 describes the penalty for such as spiritual death (and, interestingly, mentions homosexual activity as a penalty itself, a judgment for sin, not just a cause of judgment).
5 & 6 - These laws relate to how the civil society was to work. Property laws, safety, restitution, cities of refuge, etc. Obviously such things do not apply when one does not live in OT Israel.
The books of Romans (specifically Rom 13) and 1 Peter teach the student of the Bible how to live in a non-ancient-Hebrew-theocratic society.
All the examples that people usually cite (to trip up the Christian) relate to punishments prescribed for the Hebrew theocratic society, even though some of the moral violations remain moral violations today (ie, adultery).
Strictly speaking, civil laws ARE moral laws, as I've pointed out before. Any law is a moral statement.
It's wrong to go faster than 25 mph in a school zone.
It's wrong to burn down someone's property.
It's wrong to hold up a bank.
It's perfectly fine to stick a scalpel into a nearly-born baby's brain and then dismember her and 'birth' her that way.
It's perfectly fine for the gov't to force me to give them lots of the money that I earned.
It's wrong to kidnap a woman in order to protect her baby from the scalpel at the abortuary to which she is en route.
But of course, not all moral laws are civil laws, and not all civil laws are moral. It's not objectively wrong to drive 30 mph on a certain road; it is proscribed when a school is constructed on said road and concern for children's safety provokes the passing of a 25 mph speed limit law. Conversely, it is objectively wrong to murder a baby, yet that is perfectly legal in many cases in the US.
This does not mean the punishments remain or should remain the same; it means they remain violations of the moral code. Executing convicted adulterers, say, was a command for the ancient Hebrews only to follow; while adultery itself remains a moral violation, resulting in a physical/temporal/earthbound penalty (or lack thereof) to be prescribed by the society in which it is performed, it results "merely" in spiritual death and further condemnation for the one who does not have saving faith in Jesus Christ.
The civil gov'ts in countries in which Christians live make laws which Christians are obligated to follow, except when they command us to do sthg contrary to God's commandment (ie, the moral law).
So, quick review:
-Moral, civil, and ceremonial laws exist in the Mosaic Law.
-Ceremonial/purity laws are fulfilled in Christ.
-Civil laws were applicable to OT Israel, which society no longer exists.
-Moral law violations remain violations, but their penalties are in many (or even most) cases different now.
I imagine I'll be referring people back to this post quite alot, but only time will tell.