The concept of eternal life is outlined in an abbreviated form in the Old Testament (cf. Dan. 12:2). However, it comes into full bloom in the New Testament revelation. Life and immortality are brought to light through the gospel (2 Tim. 1:10).
Many believe that eternal life, as the glorious promise found in the Bible, is merely eternal existence. That is not the case. Even the wicked will exist forever (Dan. 12:2; Mt. 25:46; Rev. 14:9-12).
Eternal life is the opposite of eternal punishment (Mt. 25:46), which is called the "second death" (Rev. 20:14). As noted elsewhere, the "second death" is banishment from the presence of God and everything good (Mt. 25:41; 2 Thes. 1:9).
Accordingly, "eternal life" is everlasting communion with the wonderful and holy God (knowing him intimately -- Jn. 17:3), with his angels, and with the redeemed of the ages (cf. Eccl. 12:7; Lk. 23:43; 2 Cor. 5:8; Phil. 1:23).
Eternal life (salvation) is dependent on being obedient to Jesus Christ (Heb. 5:8-9).