[5]*The LORD saw that the wickedness of man was great in the earth, and that every intention of the thoughts of his heart was only evil continually.
(Genesis 6:5 ESV)
[9]*What then? Are we Jews any better off? No, not at all. For we have already charged that all, both Jews and Greeks, are under sin,
(Romans 3:9 ESV)
[12]*Therefore, just as sin came into the world through one man, and death through sin, and so death spread to all men because all sinned—
(Romans 5:12 ESV)
[21]*For as by a man came death, by a man has come also the resurrection of the dead. [22]*For as in Adam all die, so also in Christ shall all be made alive.
(1 Corinthians 15:21-22 ESV)
Donald K. McKim's Dictionary of Theological Terms defines heresy as, "A view chosen instead of the official teachings of a church. Such a view is thus regarded as wrong and potentially dangerous for faith." The doctrine of original sin was first articulated by the post-Apostolic father, Irenaeus in the late 2nd Century. As with many points of doctrine of that time it was not addressed earlier because there were more pressing matters church leaders had to deal with, namely the assault on the nature and person of Jesus Himself. As Gnosticism gained traction in the 2nd Century, the need to address it's attacks on orthodoxy became necessary. It is true that the most detailed polemic on original sin was put forth by Augustine of Hippo, but the doctrine did not originate with him. The fact that the infant church of Rome accepted the doctrine does not make it a Roman doctrine, no more than their belief in the Trinity or the virgin birth make them Roman doctrines.
If man is not born in a state of sin then he is born in the original state of Adam; he is born
posse peccare,
posse non peccare (able to sin and able not to sin). From creation until now, Adam is the only human being created that possessed that ability. Since Adam every human being is born
non posse non peccare (not able not to sin). Man is not in this predicament not only because he sinned, but because he is born into sin. Pelagius taught that man is born
tabula rasa (blank slate), and is not endued with original sin. To a lesser extent semi-Pelagianism mimics this belief. Is the denial of original sin heresy? Most certainly. It leaves open the window (however small) that man can achieve righteousness without Christ, and, therefore, make Christ's sacrifice of no effect - in vain.