How do you decide which parts of the Bible God is serious about?

The above reference is just good fashion sense

Easy, peasy, nacho cheesy.

If you are knowledgeable of the Bible, it's very easy to figure out.  You don't even have to think about it.  If it's in the OT (Like the example you picked), it is NOT a command.  If it is in the NT (e.g. love your enemies; Mt 5:44), it IS a command.  If you're curious about the scriptures that show this, continue reading.

The old covenant (i.e., the Old Testament*) was made only with the ancient Israelites (Ex 19:5-8; 34:27).  It was never made with the Egyptians, Assyrians, Phoenicians, or any other Gentiles.  And then the Israelites broke that covenant anyway (Jer 31:31-32).  Eventually it was superseded for all (Jew and Gentile alike) with the new covenant (Heb 7:22; 8:7,13) that was enacted after Jesus’ sacrifice on the cross (Mt 26:28).  Some sins from the old covenant were repeated in the new such as murder, adultery and homosexuality.  Some were not repeated like the prohibitions against pork, tattoos and blended fibers, and therefore are not sinful.

Some may want to judge Christians about not observing old covenant practices like the dietary restrictions, the festivals and the Sabbath, but based on Col 2:16-17 knowledgeable believers don't let them.  (It should be noted though that all of the Ten Commandments were repeated in the New Covenant except for observing the Sabbath.)

Some are confused on the issue because of a misunderstanding about “abolishing” the old law.  Jesus said, “Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them” (Mt 5:17; which Jesus did when he died on the cross; Jn 19:28; Rom 10:14).

However, his apostle Paul wrote, “For [Christ] is our peace, who made both one, and broke down the middle wall of partition, having abolished in the flesh the hostility, the law of commandments contained in ordinances, that he might create in himself one new man of the two, making peace” (Eph 2:14-15).

Is this a contradiction?  One passage says the law was not abolished, but the other says it was abolished.  At first glance it does appear to be a contradiction, but this is because the same word “abolish” is used to translate two different Greek words.  In Mt 5:17, the Greek word in question is “kataluo” which means to totally destroy (disintegrate, demolish).  In Eph 2:15, the Greek word in question is “katargeo” which has a softer meaning and means to render inactive.  Jesus did not come to totally destroy the old law.  It is scripture and thus still has usefulness (2Tim 3:16).  For instance, one would not be able to see the prophecies that were fulfilled if it were totally wiped off the face of the earth.  However, as a source for commandments, it is of no use unless those commandments are repeated in the new covenant.  In fact, for one to use it to be righteous (i.e., to be justified) in God’s eyes is to fall from grace (Gal 5:4).

If the above were not enough, Christ’s apostle Paul (Ac 9:15; 26:15-18) clearly stated that we have been “discharged from” that old Jewish law (Rom 7:6) and compared it to a tutor that led us to Christ, but now that Christ has come, there is no need for such a tutor (Gal 3:24-25).

Next time ask a hard question.

By listening to what God had to say. Jesus Christ came to Earth, clearly demonstrated that He was God by constantly doing things only God could do, founded one Church, and promised that one Church "The Holy Spirit will guide you into all truth", and "Whatsoever you bind upon Earth is bound in Heaven", and "He who hears you hears Me". That one Church teaches only what God calls it to teach. It rejects many of the manmade rules and regulations of Old Testament Judaism. And  when His Church rejects something, God rejects it.

It says the old testament laws have changed in the new after Christ era. Many old laws applied thousands of years earlier when the world was a savage tribal place. Anyway... that entire chapter is about common sense, not who goes to hell.

Looking at today's Christians. Whichever suit your own prejudices, follow them, ignore the ones that dont

Instead of mocking WHY NOT REALIZE THAT GOD KNOWS EVERYTHING while u dont??

Haha lol Jesus freaks are mentally ill

Entropy? Is that you? You are stupid aren't you? You should know the Bible before making inane assumptions that make you look childish. Are you 6? If you knew the Bible, which you don't, you would know that you are quoting a DEAD covenant. Read Hebrews and your answer will become vividly clear. But you won't will you, because all you like to do is believe and repeat lies. 

Next time you call ME a troll, I will have less kind words for you.

It's best to have living apostles and prophets in your church to get the current word of God.  New Covenant, new times, new rules.  Of course large amounts of the Old Testament are still true: the two Covenants are not mutually exclusive.

I know all gods are myths and the bible is just ancient superstition.

God is serious about it all, but a man or woman must be serious about accepting Jesus Christ as their Lord and Savior and God.  Because without Jesus, no one reaches eternal life in Heaven.  Without Jesus who God sent to take away the sins of the world, mankind has no hope.  God knew we couldn't quit sinning, so He sent Himself down in human flesh so we could have eternal life in heaven.  It is the flesh that we deal with daily and a Christian puts their flesh to death daily as we get closer to the Lord.  It is the Holy Spirit dwelling in us that gets us ready to meet the Lord.  Without the Holy Spirit we could not do it.

All of it. If I was a part of the group that was given that command, I would certainly follow it.

"GOD tells us" (I Corinthians 2:10)!