Hos 4:6 My people are destroyed for lack of knowledge. Because you have rejected knowledge, I also will reject you from being priest for Me; because you have forgotten the law of your God, I also will forget your children.
God's people are destroyed because they do not know God and His word. Apostle Paul constantly shared the knowledge, understanding and truth of God's Word from the whole Counsel of God as revealed in the Holy Bible.
Acts 20:27-30 "For I have not shunned to declare to you the whole counsel of God. Therefore take heed to yourselves and to all the flock, among which the Holy Spirit has made you overseers, to shepherd the church of God which He purchased with His own blood.
The 'whole counsel of God' will be the 'Word of God' found in both the Old and New Testaments.
In Old Testament, we understand some of these important truth:-
1. One God - revealed in the bible as Father the Creator, Son the Saviour and Holy Spirit the Comfort or Helper
2. God created the Universe - Big Bang is heresy
3. God created man in His image - filled with obedience, humility, love, compassion, grace, truth, etc. as shown in Jesus' life.
4. Fallen angels because of pride and rebellion - Satan and demons
5. Men fell to the deception of the Devil in the Garden of Eden, who caused men to disobey God.
6. Devil used fallen angels to corrupt the Human Race by producing giants (superhuman or human hybrid). God wiped them out with the deluge.
Note - Canaanites also mated with animals so God ordered the Israelites to kill all of them.
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.
7. Fallen men were wicked
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.
Since the fall of men, and with the continuous deceptions of Satan, men were not able to please God based on their own conscience. In fact, men had became very wicked. Not only were they idolaters, but were depraved, sodomites and perverts. (Please read Lev 18)
Redemption of Fallen Men
God's Plan is to restore the fallen men to how they were created before i.e the image of God. Since men failed using their conscience to please God, then God gave them covenants, for them to follow His commandments and teachings.
The initial covenant with Abraham was to get him out of Ur and then gave his descendents the promised Land in Canaan. To prevent the Israelites from idolatry, He send them into Egypt for Egyptians despised shepherds. When the time was right, God sent Moses and with signs and wonders took them out of Egypt. During the forty years in the wilderness, God taught them His Laws and covenant.
In the Old Testament we understand many attributes of God. He is righteous, just, merciful, faithful, etc. From these attributes, we can appreciate, respect and honour Him more. All false doctrines and heresies contradict either His attributes, commandments, teachings or words.
Also in the Old Testament, we understand how God judged the Israelites with the Old Covenant.
Brethren, we need to study the Old Testament so that we can understand God, His attributes, displeasures, judgements, ways, plans, etc. and also a better understanding of our New Covenant with our Lord Jesus.