Gen 6:5-6 Then the LORD saw that the wickedness of man was great in the earth, and that every intent of the thoughts of his heart was only evil continually. And the LORD was sorry that He had made man on the earth, and He was grieved in His heart.
When God created Adam and Eve, He gave them the free will to choose. But Adam and Eve chose to disobey God in the garden of Eden, and since then men started to become more and more wicked until God destroyed the earth with the deluge. The descendants of the righteous Noah did not take long to become wicked again, until God had to make the covenant with the descendants of Jacob with the Ten Commandments written on stone tablets.
Deu 5:2 "The LORD our God made a covenant with us in Horeb."
Old Testament Covenant
In the OT, the Temple of God was built in order that the Israelites can observe Moses Laws (Judaism) to fulfill their part of the covenant (esp. sacrifices and Jewish feasts). In return, God will be their God and they will be His holy people, etc.
Deu 11:26-28 "Behold, I set before you today a blessing and a curse: the blessing, if you obey the commandments of the LORD your God which I command you today; and the curse, if you do not obey the commandments of the LORD your God, but turn aside from the way which I command you today, to go after other gods which you have not known."
God required the Israelites to fulfill their part of the covenant and then He will fulfill His promises. Blessings for obedience but curses and disasters for disobedience or rebellion.
Old Testament was Unbearable
Act 15:5 But some of the sect of the Pharisees who believed rose up, saying, "It is necessary to circumcise them, and to command them to keep the law of Moses."
Act 15:8-10 "So God, who knows the heart, acknowledged them by giving them the Holy Spirit, just as He did to us, and made no distinction between us and them, purifying their hearts by faith. Now therefore, why do you test God by putting a yoke on the neck of the disciples which neither our fathers nor we were able to bear?"
Since the Jews were not able to keep the Laws, God removed Judaism by destroying the temple of God to start His new covenant of Grace in Christ Jesus out of His love for men.
New Testament Covenant
The NT is also a covenant of God so that His abundant grace in Christ Jesus will be fairly given to men, that whoever believes in Him will be saved.
John 1:17 For the law was given through Moses, but grace and truth came through Jesus Christ.
All Christians are justified by the faith in Christ Jesus. They must not follow any part of the law otherwise they will put themselves to be under the curse.
Gal 3:10-11 For as many as are of the works of the law are under the curse; for it is written, "Cursed is everyone who does not continue in all things which are written in the book of the law, to do them." But that no one is justified by the law in the sight of God is evident, for "the just shall live by faith."
Recognising Sins from the Law
Rom 7:7 What shall we say then? Is the law sin? Certainly not! On the contrary, I would not have known sin except through the law. For I would not have known covetousness unless the law had said, "You shall not covet."
However, it is from the law that we recognise sin. Christians must read the OT to recognise sins (according to God's law), and also to understand His attributes, will, thoughts, ways, powers, majesty and pleasures so that we can be able to worship, adore and please Him.