Genesis 6:3
Now it came about that when men started to grow in numbers on the surface of the ground and daughters were born to them, 2 then the sons of the [true] God began to notice the daughters of men, that they were good-looking; and they went taking wives for themselves, namely, all whom they chose.
3 After that Jehovah said: “My spirit shall not act toward man indefinitely in that he is also flesh. Accordingly his days shall amount to a hundred and twenty years.”
When man was created, he was made in God’s image, according to his likeness. (Ge 1:26, 27) This, of course, did not mean physical image or appearance, for God is a Spirit, and man is flesh. (Ge 6:3; Joh 4:24) It meant that man, different from the “unreasoning animals” (2Pe 2:12), had reasoning power; he had attributes like those of God, such as love, a sense of justice, wisdom, and power. (Compare Col 3:10.) He had the ability to understand why he existed and his Creator’s purpose toward him. Hence he, unlike the animals, was given the capacity for spirituality. He could appreciate and worship his Creator. This capacity created a need in Adam. He needed more than literal food; he had to have spiritual sustenance; his spirituality had to be exercised for his mental and physical welfare.
Consequently, apart from Jehovah God and his spiritual provisions there can be no indefinite continuance of life. As to living forever, Jesus said: “This means everlasting life, their taking in knowledge of you, the only true God, and of the one whom you sent forth, Jesus Christ.”—Joh 17:3.
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How does a person get his name permanently retained in “the book of life”?
For those who are in line to receive heavenly life, it is by ‘conquering’ this world through faith, proving themselves “faithful even to death.” (Re 2:10; 3:5) For those who are in line to receive life on earth, it is by proving loyal to Jehovah through a final, decisive test at the end of Christ’s Millennial Reign. (Re 20:7, 8) Those who maintain integrity through that final test will have their names retained permanently by God in “the book of life,” Jehovah thus acknowledging that they are righteous in the complete sense and worthy of the right to everlasting life on earth.—Ro 8:33.