It’s about Life
First, the term being defined is “life” not “eternal life”. For example, when asked about “eternal life” Christ responded about “life”:
Matthew 19:16 And, behold, one came and said unto him, Good Master, what good thing shall I do, that I may have eternal life? 17 And he said unto him, Why callest thou me good? there is none good but one, that is, God: but if thou wilt enter into life, keep the commandments.
Everyone will receive eternal life in the sense that all will be resurrected and live forever, but only those that have done good will be resurrected to “life”:
John 5:28 Marvel not at this: for the hour is coming, in the which all that are in the graves shall hear his voice, 29 And shall come forth; they that have done good, unto the resurrection of life; and they that have done evil, unto the resurrection of damnation.
“Eternal life” is also called “everlasting life”, which implies that “eternal” is the same as “everlasting” and indicates that both terms refer to a (long …) stretch of time. For example:
Matthew 19:29 And every one that hath forsaken houses, or brethren, or sisters, or father, or mother, or wife, or children, or lands, for my name’s sake, shall receive an hundredfold, and shall inherit everlasting life.
Eternal Life Is to Know Christ
Eternal life is the kind of life lived by God and Christ:
John 5:26 For as the Father hath life in himself; so hath he given to the Son to have life in himself;
1 John 1:1 That which was from the beginning, which we have heard, which we have seen with our eyes, which we have looked upon, and our hands have handled, of the Word of life; 2 (For the life was manifested, and we have seen it, and bear witness, and shew unto you that eternal life, which was with the Father, and was manifested unto us;)
We have eternal life when we know Christ intimately and thereby are living the kind of life that He lives:
John 17:3 And this is life eternal, that they might know thee the only true God, and Jesus Christ, whom thou hast sent.
Eternal Life Is to Become Like Christ
We are to learn to become eternal lives like Christ:
2 Thessalonians 2:13 But we are bound to give thanks alway to God for you, brethren beloved of the Lord, because God hath from the beginning chosen you to salvation through sanctification of the Spirit and belief of the truth: 14 Whereunto he called you by our gospel, to the obtaining of the glory of our Lord Jesus Christ.
1 John3:2 Beloved, now are we the sons of God, and it doth not yet appear what we shall be: but we know that, when he shall appear, we shall be like him; for we shall see him as he is. 3 And every man that hath this hope in him purifieth himself, even as he is pure.
Eternal Life Is Fully Realized in the Next World
Eternal life will be fully realized in the world to come:
Mark 10:28 Then Peter began to say unto him, Lo, we have left all, and have followed thee. 29 And Jesus answered and said, Verily I say unto you, There is no man that hath left house, or brethren, or sisters, or father, or mother, or wife, or children, or lands, for my sake, and the gospel’s, 30 But he shall receive an hundredfold now in this time, houses, and brethren, and sisters, and mothers, and children, and lands, with persecutions; and in the world to come eternal life. 31 But many that are first shall be last; and the last first. (see also Luke 18:28-30)
Realizing Eternal Life Requires Endurance
The learning/growth process must be completed to be an eternal life:
Mark 13:13 And ye shall be hated of all men for my name’s sake: but he that shall endure unto the end, the same shall be saved. (see also Matthew 24:13)
It is possible that some who have realized the work of the Spirit in their lives may turn away and be lost.
Hebrews 6:4 For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, 5 And have tasted the good word of God, and the powers of the world to come, 6 If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.