The original plan of God when He created man was that man should have dominion over all other creatures and the entire universe.
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Adam and Eve who were created in the image and likeness of God we're expected to be fruitful and multiply; to fill the earth and subdue it. With this perfect desire of God every human being coming from Adam would have automatically become a child of God - Genesis 1:26-28.
However that was not to be because Adam and Eve floated the only commandment God gave them in the garden. By succumbing to the Devil, the tempter, Adam and Eve had unwittingly sold mankind to the evil one. It was not surprising that soon after the fall of man the first murder recorded in history was committed when Cain, the first son of Adam, killed Abel his younger brother. Thus began the chequered history of mankind with more failures than success. With the Devil in control, the world began to wallow in sins, violence and wickedness - Genesis 3:17-19; 6-5-6.
This was the situation in the world for about 2000 years till the time of Noah when God destroyed with flood all human beings on earth except Noah and family of eight souls. Hundreds of years later God called Abraham to a covenant relationship. God promised to redeem mankind through the seed of Abraham. Between this promise and it's fulfillment in the death and resurrection of Jesus Christ spanned another 2000 years. Throughout the Old Testament era which spanned through 4000 years, there were people in the minority who were faithful to the Living God. These are Old Testament saints such as Abraham, Isaac, Jacob, Joseph, Samuel, David, Hezekiah, Jeremiah, etc who through faith got connected to the Holy One of Israel. By faith they kept the statutes of the Almighty God and when they sinned, they shed the blood of animals for the remission of their sin - Hebrew 1:1-2.
However out of uncommon love that God had for mankind, He sent His only Son, Jesus Christ, to die for the world in order to redeem man back to Himself - John 3:16.
Paul the Apostle in reference to this situation wrote that "All have sinned and have fallen short of the glory of God" and also that "The wages of sin is death but the gift of God is eternal life through Jesus Christ"- Romans 3:23; 6:23.
Even though Christ died for all human, it is only those whosoever repent of his sins and believe in His sacrifice on the cross by faith that will be forgiven and become a child of God. Jesus came unto His own, but His own received Him not. But as many as received Him, to them gave He the power to become the sons of God -John 1:11-12. It is by the grace of God through Christ that we have the chance of becoming adopted son's of God - Romans 8:15.