The Fourth Gospel is based on the positive and enhancing gift from Jesus of Life Everlasting in Heaven, with everyone who chooses likewise; offered to all those who love, trust, believe-in and follow Jesus.
(And therefore, naturally, live accordingly.)
But because of the historical ignoring or subordination of the Fourth Gospel, and doctrines and practices based on the Synoptic Gospels, the Epistles and other parts of the Bible, or church authority, or tradition, or philosophy... for most of its history nearly all of Christian evangelism has been based on a double-negative strategy.
The double-negative strategy is to state that Christianity will save Men from something horrible.
So, without Christianity Men face negatives such as fear of Hell, extinction, misery, futility, guilt; and the message is that Jesus came to save us from these negatives.
But this no longer works in the modern world because our culture is psychological, and the negatives are regarded as psychological symptoms. So the negatives of life (fear, despair, guilt etc) are seen as emotional states; and the strategy for dealing-with them is this-worldly therapeutic.
Modern culture offers to 'save' us from such emotional states (either now or in the future) by essentially technological means such as politics, sex, drugs and distractions. If we are afraid, we take a tranquilliser; if we are miserable we take an antidepressant or get drunk, if we feel guilty we take our minds off the problem by immersion in mass and social media... etc.
However, if (like me) you regard the Fourth Gospel as the heart and true message of Christianity; then Christians can offer something that the secular culture has completely failed to address; which is an escape from nihilism, despair and alienation; or, to put it positively - the provision of meaning, purpose and hope.
And the message and lesson is simple: If you want this - life eternal, as divine beings participating in God's creation - then You Can Have It.
(But... To have it You Must Really Want It.)
Eternal Life does not come to everybody, because we are free agents and Heaven is the kind of condition must actively be chosen; and most modern people (apparently) don't want it and reject it.
You must want Heaven more than anything else.
And that's all.
(But for most people, that is way too much...)