Those wonderfully twisted folks at The Lutheran Satire explain the distinction between the moral law and the ritual purity law to internet atheists. It is an important issue and I have addressed it before:
A question arose yesterday in a thread, posed by Michael:
I have a real question. Homosexuality, as a sin an abomination, is mentioned in Leviticus. That book, however, also says:
– disrespect of parents should be punishable by death
– sleeping with a woman during her period should make both parties outcasts
– don’t eat pork
– shellfish are an abomination
So my question is, why are some of the verses ignored and others so important?
It is a good question and sometimes confuses Catholics and non-Catholics. The answer to the question is in the very earliest history of the Church. After the ascension of Jesus, the apostles went about the great task of making “disciples of all the nations”, and Christianity began to spread among Jew and Gentile alike. The question quickly arose as to whether Gentile converts would have to be circumcised (the males only of course!) and follow all of the Jewish laws regarding ritual purity. If they were asked to do this, it would mean a complete revolution in their life. They would no longer be able to even eat a meal with their Gentile relatives and friends. Like the Jews, the Christians would be a people set apart, cut off from interacting in the simplest ways with non-Jews for fear of violating the hundreds of laws of the Old Testament regarding ritual purity.
From the start, Saint Paul viewed it as entirely improper to make Gentile converts live like Jews. He understood that what he had once valued as a strict Pharisee was as nothing in the light of Christ. Saint Peter, the head of the Church, initially temporized, eating freely with Gentile converts, but adhering to Jewish customs when around Jewish converts. Saint Paul pointed out the flaws in this bandage over the problem. Saint Peter eventually had a vision in which he was told by God to eat various animals deemed unclean for Jews in the Old Testament, and then understood that Christ was leading all Christians, Jew and Gentile alike, to a new way of life.
Tensions over the issue of whether Gentile Christians should be required to follow all of the Jewish ritual purity laws, were settled doctrinally once and for all at the Council of Jerusalem in 49 AD which issued these decrees:
20But that we write unto them, that they abstain from pollutions of idols, and from fornication, and from things strangled, and from blood.
21For Moses of old time hath in every city them that preach him, being read in the synagogues every sabbath day.
The Gentiles were therefore not to be required to live like Jews. They were required to follow the ten commandments and the major teachings of the Old Testament, but the laws of ritual impurity were not to apply to Christians. The working out of this simple concept took centuries, and the Epistles of Saint Paul amply demonstrate the battle that roiled within the Church over this question, Saint Paul’s frustration boiling over in Galatians 5:12 where he wishes that those insisting on the circumcision of Gentile converts would castrate themselves. In Galatians Chapter 5, Saint Paul also makes clear what is binding on Christians in regard to conduct:
 And I, brethren, if I yet preach circumcision, why do I yet suffer persecution? Then is the scandal of the cross made void.  I would they were even cut off, who trouble you.  For you, brethren, have been called unto liberty: only make not liberty an occasion to the flesh, but by charity of the spirit serve one another.  For all the law is fulfilled in one word: Thou shalt love thy neighbour as thyself.  But if you bite and devour one another; take heed you be not consumed one of another.
 I say then, walk in the spirit, and you shall not fulfill the lusts of the flesh.  For the flesh lusteth against the spirit: and the spirit against the flesh; for these are contrary one to another: so that you do not the things that you would.  But if you are led by the spirit, you are not under the law.  Now the works of the flesh are manifest, which are fornication, uncleanness, immodesty, luxury,  Idolatry, witchcrafts, enmities, contentions, emulations, wraths, quarrels, dissensions, sects,
 Envies, murders, drunkenness, revellings, and such like. Of the which I foretell you, as I have foretold to you, that they who do such things shall not obtain the kingdom of God.  But the fruit of the Spirit is, charity, joy, peace, patience, benignity, goodness, longanimity,  Mildness, faith, modesty, continency, chastity. Against such there is no law.  And they that are Christ’s, have crucified their flesh, with the vices and concupiscences.  If we live in the
Spirit, let us also walk in the Spirit.
 Let us not be made desirous of vain glory, provoking one another, envying on another.
So that, in a nutshell, is why portions of the Old Testament are binding on Catholics and other portions are not.
– See more at: https://www.the-american-catholic.com/2011/09/01/council-of-jerusalem/#sthash.ZzmFkVXz.dpuf