In evangelism you are right in starting with your friend's conviction and in the process you have successfully identified a major concern: that he is satisfied with little happiness. Your deployment of the cave allegory is thus appropriate but you need to show how staying in the cave is ultimately life threatening.
Various adaptations of Plato's Allegory of the Cave
This is where Platonism differs from Christianity. If we use the original meaning in Platonism (where this allegory originated), it's an allegory for our laziness to stay within the material realm. It's painful to pursue philosophical truths (the sun gets in our eyes), but it's the only way forward for enlightenment. As the allegory was originally conceived, however, it was meant to be optional: to pursue philosophical truths / enlightenment when one is ready, usually at retirement from active life. This meaning is still true for Christians today (especially for those who becomes monk / hermit or who undertakes the path of asceticism & mysticism). But Christianity does NOT require one to pursue this path in order to be saved. In this sense, your friend is right, why bother with hard truths when there are so many visible things to take care and that can bring us earthly happiness (which God declares "good"): family, career, wealth, health?
I understand you try to modify this allegory by making it to mean theological and spiritual truths that are necessary from the young age (cf. Ecclesiastes) so we can be on the safe paths until we arrive in heaven. We all agree (as well as your friend) that these truths that can make us enemies of those who would rather stay in the cave. In other words, you are trying to modify this allegory to mean carrying Jesus's cross to imitate Jesus, which brings us suffering, mockery, and potentially self-chosen denials of some earthly happiness / pursuit.
C.S. Lewis has been instrumental for me to embrace Christianity. He also deploys the allegory of the cave (although not in the way you used it) in his 4th Narnia book, The Silver Chair (see journal article The Silver Chair and Plato's Allegory of the Cave: Archetypes of Spiritual Liberation). The whole Narnia series itself is a gentle allegory to view Christianity from a fresh perspective. But in order for it to be effective, it's necessary to read the whole series, something that may not work for your friend.
How to motivate someone that something outside the cave is worth pursuing
Evangelism is a success when the person decides to take up the cross and follow Jesus. At this point your friend seems far from ready, short of God's direct intervention through a crisis, etc. Even Jesus warned us to count the cost.
Therefore, we need to first start by showing the consequences of taking no action:
- A good first step then is to show what your friend will be missing by staying in the cave. This is the famous C.S. Lewis argument from desire, which he uses in his very famous essay The Weight of Glory.
- Then you can show the nature of true happiness in God. One way is by process of elimination, using St. Thomas Aquinas's famous discussion of The Ultimate End of Human Beings. This is an argument from design: that there are inherently certain spiritual desires proper to man that can only be fulfilled by God.
How to "sell" Christianity as the way to ultimate happiness (despite the cost)
Then we need to clarify Christianity's position in 2 areas important to your friend (freedom and happiness) and add 2 important elements: morality and new life. This is the general trajectory I find useful:
Platonists, Aristotelians and Stoics teach that:
- happiness IS acting virtuously
- immaterial souls freed from this "bothersome" body give us better happiness.
But common sense, C.S. Lewis, and Christianity disagree with them on both points:
- happiness (or simply, pleasure) is not always the result of virtuous activity
- bodily resurrection is necessary for eternal happiness in the next life (God declares that bodily existence is "very good").
What is the connection between morality and happiness? It is extremely important to note that in Christianity acting virtuously (living a moral life, defined as obeying the voice of conscience planted by God in our nature) does NOT automatically lead to happiness (defined as pleasure and opportunities / resources to experience pleasure). Happiness is a gift (God's reward), which can be experienced starting here, but will only be fulfilled incorruptibly in heaven. Before the world ends, God lets the sun shines on the righteous and the wicked alike, which we can take to mean that God gives happiness in various measures to everyone whether they deserve it or not. Hence the Psalmist's many complaints to God that how come the righteous suffer but the wicked prosper, and the Psalmist's pleas that God should to do something about it soon. So the main salvation question is: how to ensure that God does not take this temporary happiness from us but instead rewards us with eternal happiness in the life to come. The answer: obtain new life (see the next point).
What's the connection between happiness, new life, and morality?
- Obtaining new life is the gospel proper, the gateway for eternal life and ultimate happiness. To obtain new life we first need to be declared righteous (justified).
- Christianity is adamant that we are declared righteous by grace only: it's 100% gift and not through morality but only through faith in Jesus Christ. Some denominations require baptism.
- After receiving the new life, our moral life becomes different. It's no longer our attempt to placate / curry favor with God or to neutralize karma, but a response to this gift. The moral life also becomes easier, empowered by grace through this new life.
- The new life is both a source of joy (different than happiness !!) but which can cause us suffering in the world (hence Jesus's warning). We can be joyful despite worldly unhappiness because we have righteous standing before God, which gives us the status of His adopted son/daughter, which in turn gives us peace (in NT sense) since God, the most potent being in the universe, is no longer our enemy. This status enables us to approach God as our friend and as our perfect father whom Jesus pictured beautifully in the gospels.
What is the connection between new life and freedom? In Christianity, true freedom has several meanings and some of them are obtained as benefits of having a new life when we become Christian:
- freedom from the power of sin which frustrates our attempts to live virtuously (see my answer on Rom 7:24)
- freedom from cults / false teaching / false religions that impose unneeded requirements / greater burden on us ("my yoke is easy and my burden is light")
- freedom to approach God as an adopted son in joy and peace, like a child to a perfect Father. We have access to him when we are in dire situations because of worldly enemies / sufferings / needs.
- freedom from chasing worldly goods obsessively but only as needed, i.e. we just need enough wealth, health, honor, & power to fulfill Jesus's plan for us
- freedom to live to the fullest within a Christian worldview, so to avoid being enslaved by any part of it ("the truth shall set you free")
- freedom to from the world's attempt (especially in totalitarian states) to reduce the fullness of our human nature (and the rights that are associated with it) by the world's denying / distorting some parts of our nature, which is made in the image of God
The 4 points above are the "key value propositions" important to your friend that Christianity has to offer. The hardest selling point seems to be whether the joy, peace, greater freedom, and promise of ultimate happiness (in the next world) is enough enticement to offset the cost: the promised life of suffering in this world. Even in the best scenario where there is not much external suffering outside the cave, there will be much internal suffering in living a better moral life, because as true Christians we will become more aware of our sins and consequently suffer with remorse and with the hard work to purge those remaining sins within us.
Also, I would make a "full selling disclosure": that happiness in this world is not guaranteed. I was quite shaken when I learned how God took away C.S. Lewis's wife from him despite how C.S. Lewis lived an exemplary Christian life from what I learned through his biographies and letters (see the beautifully made movie Shadowland (1993) despite two major inaccuracies). This underscores my conviction that happiness is a gift, not a right obtained through moral living pleasing to God.
The movie shows 3 Christian truths of life outside the cave that are hard to swallow although they lead to imperfect happiness now and heaven in the future:
- Love brings happiness, which is earthly glimpse of heaven. Quote:
“I love you, Joy. I love you so much. You made me so happy. I didn't know I could be so happy.”
- But in order to love you have to abandon safety. Quote:
“Twice in that life I've been given the choice: as a boy and as a man. The boy chose safety, the man chooses suffering. The pain now is part of the happiness then. That's the deal.”
- Suffering is God's way to shape us to fit for heaven. Quote:
“I suggest to you that it is because God loves us that he gives us the gift of suffering. Pain is God's megaphone to rouse a deaf world. You see, we are like blocks of stone out of which the Sculptor carves the forms of men. The blows of his chisel, which hurt us so much are what make us perfect.”
Once your friend believes that becoming Christian is the only way to obtain new life, the last step is for your friend to take action. Otherwise you'll need to take additional apologetics steps by showing the trustworthiness of the Bible, how prophecies were fulfilled in Jesus, etc.