Eternal life is the life believers receive from God through Jesus Christ — not merely endless duration, but present and everlasting participation in the life of God.
To have eternal life is to know God and Jesus Christ whom He has sent; it is a quality of life as much as a length, begun the moment a person believes and never ending. He that hath the Son hath life.
This life is received, not achieved, and held by union with Christ. Because it is His risen life shared with His people, death cannot sever it, and bodily resurrection will complete it.
See in Scripture: John 3:16; John 5:24; John 17:3; Romans 6:23; 1 John 5:11–12