Pelagianism
Pelagius was a British born 5th Century monk who taught in Rome that man had the moral/natural ability to come to God on his own. By claiming that man had the same purity and moral ability as pre-fall Adam, Pelagianism denies original sin, the depravity of man, and the substitutionary atonement.
Pelagius was confronted by Augustine of Hippo, Pelagianism was condemned as heresy at the Council of Carthage in 418 AD, and once again at the Council of Ephesus in 431 AD.