In Christianity, the phrase 'original sin' refers to the concept that Adam's sin in Genesis 3 resulted in a 'fall' of all humanity. Adam's sin corrupted his nature, his very being, and he passed that corruption on. All humans are 'born sinners'; they are born under the condition of Adam's sinful nature, and so are condemned by default of existing.
This concept persists in nearly all forms of Western Christianity, but it has been challenged on multiple fronts.
From one moral approach, many find the idea that all humans are responsible for Adam's guilt to be contrary to the repeated declaration in the bible that no one is responsible for someone else's sins. Such a thing would be unjust. These people retain the idea of a 'sin nature', that Adam 'infected' humanity as a whole with a corrupted nature that inclines everyone toward sin, but they reject the idea that Adam's guilt has been imputed upon his descendants.
From another moral approach, many others find the notion that children may be condemned for having this sin nature to be even worse. Children are ignorant, they have no awareness of 'sin', so how can they be held responsible for whatever sins they might commit? So these people craft a balancing doctrine called 'the age of accountability': children are innocent of their sins until they reach an age when they can understand what they're doing.
And further still, from a scientific approach, the widespread acceptance of biological evolution as a fact poses a simple problem: not all humanity is descended from a single man. Even if the idea of the 'sin nature' is true, and even if Adam was a literal human individual living a few thousand years ago, not all humanity would be descended from him, so not all humanity would have this 'sin nature'.
These are points Christians should consider if they insist on the doctrine of 'original sin', but they are not what I want to address. My concern here is the textual origin of this doctrine.
From a historical standpoint, the first person to articulate this idea of 'original sin' and the consequent 'sin nature' was Augustine, in the early fifth century AD. Augustine wrote:
The apostle, however, has declared concerning the first man, that 'in him all sinned' […] that is, who had not yet sinned of their own individual will, as Adam did, but had drawn from him original sin, 'who is the figure of him that was to come,' because in him was constituted the form of condemnation to his future progeny, who should spring from him by natural descent; so that from one all men were born to a condemnation
Augustine, On Merit and the Forgiveness of Sins 1.11-13
Augustine wrote in Latin, but barely knew Greek, so he relied on a Latin translation of biblical texts like Paul's letter to the Romans. The verse from which all of Augustine's idea of 'original sin' was derived is Romans 5.12, which in Latin said, in quo omnes peccaverunt. This translated the Greek phrase εφ ω παντες ημαρτον.
The issue here is that Augustine was working with an unclear Latin translation of Romans, resulting in a severe misinterpretation of what Paul actually wrote.
Augustine wrote that every human suffers death because we inherit guilt from Adam:
Therefore, just as sin came into the world through one man, and death came through sin, and so death spread to all; in him all sinned (in quo omnes peccaverunt)
Paul wrote that every human suffers death because we have our own guilt:
Therefore, just as sin came into the world through one man, and death came through sin, and so death spread to all because all have sinned (εφ ω παντες ημαρτον)
Augustine says people are condemned for being; their existence is marred by sin, so everyone deserves to be punished for Adam's guilt.
Paul says people are condemned for doing; their existence is not corrupted, their immoral choices are the problem.
Augustine's idea of sin is completely unrecognizable to the Jewish idea of sin. Judaism doesn't think of sin as a nature humans have, as a condition we're born in, or whatever. Sins are immoral choices. It's something we do, not something we are.
In contrast to Augustine, Paul was deliberately writing in broad strokes, grouping all of humanity alongside Adam so that he could set up his typology between 'Adam' (us) and Jesus. The earthly, mortal 'Adam' (us) is brought to life by the heavenly, immortal Jesus. (What Paul expresses in Romans 5 is identical to what he writes in 1 Corinthians 15, when talking about resurrection.)
Paul was expressing traditional Jewish idea of sin, but he was doing it with sweeping, apocalyptic language. When we look at another apocalyptic text from the period, written by a Jew who didn't follow Jesus, we find he wrote about sin exactly the same way as Paul did:
Therefore, just as sin came into the world through one man, and death came through sin, and so death spread to all because all have sinned […] because of the one man’s trespass, death exercised dominion through that one
Romans 5 (c AD 58)
For although Adam sinned first and brought untimely death upon all humanity, each of those who has been born from him has prepared for his own soul the coming torment. […] So Adam is not the cause, with an exception of his own soul, but each of us has been the Adam of his own soul.
2 Baruch 54 (c AD 100)