Well, it didn't take much looking and here is the pertinent quote that defines Naturalism, which is actually a precursor to Pelagianism (a more commonly known heresy. This is taken from an online Archive of Fr. John A. Hardon SJ's works- what a treasure trove!
"Pelagianism against the Necessity of the Redemption. The principal heresy of naturalism in Christian history was born of Stoic philosophy that infected certain writers from apostolic times. Yet, the full-blown system of Pelagianism into which it developed did not arise until the beginning of the fifth century. A British lay-monk, Pelagius, first popularized the theory, together with his disciple Celestius. Little is known about the life of Pelagius, except that he was born in England about 354 A. D. (the year of Augustine's birth) and came to Rome where he became alarmed by the low morality of priests and people. He concluded that the only hope of reform lay in placing all the responsibility for sin on the free wills of men, to the point of denying the necessity of Christ's redemption or of divine grace.
The premises served as basis for Pelagius' thesis. Arguing from the principle that "a person is free if he does what he wills and avoids what he wills to avoid," he said that heaven is attainable by use of our natural faculties alone, since nothing but the free will is needed to practice virtue and keep out of sin." [emphasis mine]
So basically, anyone who thinks they can get to heaven on natural goodness is wrong. You must keep the Faith of the Catholic Church, whole and entire.