The mysteries of the Kingdom of God
Eternal life is one of the mysteries of the gospel, of Christian revelation. We do not find this term in the old testament, instead we find the term Son of Man, in the book of Ezekiel.
In the old testament in the book of wisdom there is a term that has an similar use to eternal life: "Immortality"; "in the familiarity with the wisdom is the immortality" Wisdom 8:17.
While Immortality is a divine attribute, in the context in which it is used the term it can be seen as a "fruit of the spirit" and the same thing happens with eternal life in the Gospels.
Jesus Christ teach us: "This is eternal life, that they may know you only true God" John 17: 3. Eternal life is here and now, it is a spiritual gift and fruit of the spirit. It is a fruit of the spirit because of the bliss: "Blessed are the pure in heart, for they shall see God", Matthew 5:8. And it is also a spiritual gift because it is a gift that is accessed by grace, remember the expression: "because the wages of sin is death, but the gift of God is eternal life", Romans 6:23 We receive this gift when we are born again.
Thomas Aquinas raises an answer: eternal life is contemplative activity. This is a close definition but not very precise. We know God through contemplation.
Aristotle tells us: "Happiness is the reward for virtue", and since eternal life is the most perfect virtue, we can also say that eternal life is a perfect form of happiness: happiness of the heart, happiness as a state of the mind.
In other words, to be truly happy is to know God. Also, meditation is associated with the improvement of health, the long life.
And being the contemplation a transcendental meditation, it is logical that contemplation is associated with longevity, the gift of health.
Eternal life is not only happiness and contemplation, it is a set of blessings that perfect the life in God's grace.