Much philosophical and religious theorizing is fueled by the sense that humanity is in some sort of predicament, or condition, from which we ought to be liberated in order to truly flourish. A world view is partly distinguished by how it characterizes this predicament -- what we could call its diagnosis. The nature of the diagnosis will of course largely determine the proposed cure, or means of liberation -- what is sometimes called a worldview's soteriology (from the Greek soteria = 'deliverance') Platonism is no exception to this rule. Most have heard of Plato's memorable 'allegory of the cave', which expresses an understanding of the human predicament that would shape Western philosophy for centuries to come. Recall, humans are likened to prisoners in a cave, who can only see the shadows cast on the cave wall in front of them. One of the prisoners somehow escapes the cave and sees the outside world. Though the sunlight initially hurts his eyes, he is amazed at what he sees -- the sun, sky, water, trees etc -- and judges it to be far superior to the world down below. He quickly returns to his fellow cave-dwellers, to tell them of the other world he has just seen. They aren't convinced and brush him off, and some even try to kill him. As is well known, the cave wall is meant to symbolize the world as it directly appears to us, while the outside world is meant to symbolize the world as it really is -- in particular, the perfect, immutable forms. The prisoner who escapes and sees the outside world represents the philosopher, who, not content with the immediate deliverances of the senses, wants to see the world for what it is: a perfect, harmonious kosmos governed by an intelligent mind. What really matters for our purposes is that, for Plato, the human predicament is epistemological. That is, our problem is that we lack knowledge of the world as it really is. This profound ignorance prevents us from accessing the happiness and peace that come with the unhindered contemplation of the good. This perspective of the human condition dominated Greek philosophy since Plato. The Stoics, for instance, famously held that suffering is a consequence of a failure to understand that the kosmos is fundamentally good, and hence that 'evils' like the death of loved ones are really meant to be. Interestingly, the notion that ignorance is the root cause of the human predicament is also very widespread in Indian philosophical traditions, in which knowledge of reality 'as such' (jñāna) is typically deemed crucial to the achievement of liberation from suffering (moksha).
St Paul the Apostle, the most prominent New Testament author and first great Christian theologian, like Plato, believes that humanity is mired in an unfortunate condition, from which it ought to be freed. His diagnosis is laid out in the first chapter of his magnum opus, the epistle to the Romans. A cursory reading of this chapter should make it clear that Paul's diagnosis of the human predicament is very different to Plato's. Notice that Paul explicitly denies that our problem is fundamentally about ignorance. To the contrary, he tells us that we already have knowledge of ultimate matters, or at least have easy access to them:
"Ever since the creation of the world his eternal power and divine nature, invisible though they are, have been understood and seen through the things he has made"
The passage goes on to say that though humans 'knew God, they did not honor him as God or give thanks to him". Paul's use of words is important here. 'Honor' and 'give thanks' have to do with your attitude towards someone. The term 'honor', however outdated it seems today, refers to the kind of humble reverence that is owed to, say, a parent, a teacher, or perhaps even an older, wiser friend. To 'give thanks' is to gratefully recognize someone's generosity. An attitude of gratefulness and deep respect is a necessary precondition for a healthy relationship with someone whose responsibility it is to look after us (again, like a parent or teacher). Failure to (sincerely) honor and give thanks to God, then, can only mean that our relationship with God will fail as well. In short, our predicament, for Paul, is fundamentally relational rather than epistemological: humanity has somehow collectively alienated itself from God. The result of that, according to Paul, is that humans are "futile in their understanding", arrogantly "claiming to be wise" despite having become "fools" who worship idols or "images". God's initial response is to "give them up" to their madness, which further exacerbates their alienation, and leads them to be alienated even from one another (as they become'selfish', 'murderous' 'slanderers' and so on).
This take on the human condition is of course not original to Paul. In fact, Paul's account reveals his deeply Jewish mindset, since it really retells the Ancient Jewish narrative of the 'Fall of Man'. Human corruption and suffering comes into the world when Adam and Eve break fellowship with God, preferring to be their own gods. This pattern is repeated throughout the Hebrew Bible, not least in the Prophets, where oppression and injustice invariably arise when Israel rejects its covenant with God in order to pursue idols of their own making.
To sum up, then, for Plato, humans do not truly flourish because they are ignorant of the divine. It is quite telling that Plato rejected the possibility of akrasia, i.e. acting against your better judgment. If you really know the good, he tells us, then, necessarily, you will follow it.
For Paul, humans do not truly flourish because they do not have fellowship with the divine. Against Plato, Paul directly affirms the possibility of akrasia: "For I do not do what I want, but I do the very thing I hate (...) For I do not do the good I want, but the evil I do not want is what I do." (Rom. 7:14-20). To put it informally, ours is a 'heart' rather than a 'brain' problem.
As I said at the start, different diagnoses suggest different cures, or soteriologies. For Plato, the cure lies in ridding ourselves of our ignorance through the philosophical contemplation of the perfect forms. This consists in a kind of recollection (anamnesis) since, for Plato, the soul had direct access to the forms prior to being embodied in the physical world. Hence it is fair to say that Plato and his followers advance an intellectualist soteriology. For Paul, since our predicament is a lack of fellowship with God, the cure lies not in contemplation but in reconciliation (what theologians call atonement, literally "at one"-ment). This is a point on which his fellow Jews would have agreed. Paul parts company with his more conservative Jewish brethren when he declares that this reconciliation is somehow achieved through the life, death and resurrection of Jesus of Nazareth, the incarnate Son of God: "God was reconciling the world to himself in Christ, not counting people’s sins against them. And he has committed to us the message of reconciliation." (2 Cor. 5:19).
An application: Christians sometimes present salvation as simply a matter of understanding and accepting a set of religious doctrines. In doing so, we are perhaps being more Platonic than Paulinian. An observation: it speaks volumes that platonistic pseudo-Christian cults in the Ancient World called themselves 'gnostics' (gnosis = 'knowledge') and held that liberation is attained by acquiring esoteric knowledge of the divine.