Gen 6:4 There were giants on the earth in those days, and also afterward, when the sons of God came in to the daughters of men and they bore children to them. Those were the mighty men who were of old, men of renown.
When Satan corrupted the human race, God destroyed all humans with the deluge. Only Noah's family was preserved by Him to continue to propagate the pure human race created in His image.
Gen 6:5 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.
It only took about ten generations after the deluge that almost all the descendents of Noah were wicked and idolatrous. God had to call Abraham to leave Ur from his idolatrous family.
His son Isaac took an idolatrous wife Rebecca (sister of idolatrous Laban) and his grandson Jacob too took another idolatrous wife, Rachel (daughter of Laban).
To prevent the descendents of Abraham from becoming idolatrous, God sent all of them to Egypt. Since the Egyptian did not allow shepherds (Hebrews) into their homes and temples, the Israelites were preserved as monotheist.
As soon as Moses led them out of Egypt into the Promised Land, which has was filled with giants (human hybrid - descendents of fallen angels and women), God made a covenant with the Israelites by giving them the Ten Commandments and Mosiac Law to help them to be free from idolatry and wickedness.
It did not take long after the death of King David, that his son King Solomon (who built the temple of God) was influenced by his wives to idolatry. Both the kingdoms of Israel and Judah were idolatrous and rebellious until God sent them to Babylon as captives.
After their return from captivity, once again they were rebellious until our Lord Jesus scolded the Jewish leader as hypocrites. (Please read Mat 23). They crucified their Messiah because they were faithless, and feared man (Romans) more than God.
As another human, we can associate ourselves with the Israelites. Unless our faith in Him is anchored on His words and truth, it will not take long before unbelief in Him and the Bible sets in. Just like the Israelites, we will be defiled with disobedience, sins and worldly lusts.
The Old Testament
It is in the Old Testament (through God's prophets and revelations) that His attributes, pleasures, characters, will, ways, rewards and judgment were shown to the Jews (Israelites who became disciples of Judaism or Mosiac Law).
Mosiac Law also listed many sins with some deadly sins, which they must avoid. We must read the Law to know and understand these sins.
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."
Heb 13:8 Jesus Christ is the same yesterday, today, and forever.
From the OT, we understand His displeasures, chastisement and judgment of sins, His faithfulness to His words and promises, His forgiveness and restoration, His joy and pleasure at our repentance and obedience, His rewards and gifts at our obedience, His help in our walk to holiness and godliness, His love and patience in transforming us into His image, etc.
With this knowledge, our walk with Him will be filled with light for we can see what is ahead of us. We can rejoice in both sufferings and happiness for our loving Father is in total control of our lives.