Introductory Reading: Ecclesiastes 3
"For everything there is a season, and a time for every purpose under heaven: a time to be born, and a time to die... For that which befalleth the sons of men befalleth beasts; even one thing befalleth them: as the one dieth, so dieth the other; yea, they have all one breath; and man hath no preeminence above the beasts: for all is vanity. All go unto one place; all are of the dust, and all turn to dust again. Who knoweth the spirit of man, whether it goeth upward, and the spirit of the beast, whether it goeth downward to the earth?..."
"Mortal"
Cambridge Dictionary defines "mortal" as
(of living things, especially people) unable to continue living for ever; having to die:
The Bible teaches that man is mortal:
Psalm 6:5 For in death there is no remembrance of Thee; in the grave who shall give Thee thanks?
Psalm 115:17 The dead praise not the Lord, neither any that go down into silence.
The Christadelphian Statement of Faith (below) has summarised the topic of the nature of man, the reason for death, the hope of reconciliation to God (our creator) and the paired promise of a hope beyond death - in God's kingdom on earth led by Jesus Christ as King. We'd encourage you to explore the Bible references that accompany each of these excerpts.
Christadelphian Statement of Faith Regarding Man's Nature and the Hope for the Future
4.— That the first man was Adam, whom God created out of the dust of the ground as a living soul, or natural body of life, “very good” in kind and condition, and placed him under a law through which the continuance of life was contingent on obedience (Gen. 2:7; 18:27; Job 4:19; 33:6; 1 Cor 15:46-49; Gen. 2:17).
5.— That Adam broke this law, and was adjudged unworthy of immortality, and sentenced to return to the ground from whence he was taken — a sentence which defiled and became a physical law of his being, and was transmitted to all his posterity (Gen. 3:15-19,22,23; 2 Cor. 1:9; Rom. 7:24; 2 Cor. 5:2-4; Rom. 7:18-23; Gal. 5:16,17; Rom. 6:12; 7:21; John 3:6; Rom. 5:12; 1 Cor. 15:22; Psa. 51:5; Job 14:4).
6.— That God, in His kindness, conceived a plan of restoration which, without setting aside His just and necessary law of sin and death, should ultimately rescue the race from destruction, and people the earth with sinless immortals (Rev. 21:4; John 3:16; 2 Tim. 1:10; 1 John 2:25; 2 Tim. 1:1; Titus 1:2; Rom. 3:26; John 1:29).
19.— That God will set up a kingdom in the earth, which will overthrow all others, and change them into “the kingdoms of our Lord and his Christ” (Dan. 2:44; 7:13,14; Rev. 11:15; Isa. 32:1,6; 2:3,4; 11:9,10).
20.— That for this purpose God will send Jesus Christ personally to the earth at the close of the times of the Gentiles (Acts 3:20,21; Psa. 102:16,21; 2 Tim. 4:1; Acts 1:9,11; Dan. 7:13).