The person who opened the church door wide open to all sorts of error was the Bishop of North Africa, Augustine of Hippo as he departed from the early church teachings on a number of issues and has been one of the greatest influences on the church since, both Catholic and Protestant. Luther and Calvin were the other two, both agreeing with Augustine.
The most significant was his departure from the Free Will doctrine from all the early church fathers to the Deterministic theology of Gnosticism.
“Men are possessed with free will, and endowed with the faculty of making a choice. It is not true, therefore, that some are by nature good, and others bad.” (Irenaeus Against Heresies Book IV, Chapter XXXVII)
“…those ancient writers, in general, say that Manichaeans denied free-will. The reason is, that the Fathers believed, and maintained, against the Manichaeans, that whatever state man is in he has the command over his own actions, and has equally power to do good or evil.” (Beausobre, The Christian Examiner, Volume One, Publishded by James Miller, 1824, Edition, p. 70)
Lyman Beecher said, “…the free will and natural ability of man were held by the whole church… natural inability was to that of the pagan philosophers, the Gnostic’s, and the Manichaeans.” (Lyman Beecher, Views in Theology, Published by Truman and Smith, 1836 Edition, p. 56)
“No reward can be justly bestowed, no punishment can be justly inflicted, upon him who is good or bad by necessity, and not by his own choice.” Tertullian (Doctrine of the Will by Asa Mahan p. 61, Published by Truth in Heart)
Ignatius was another figure in the Early Church. He was a disciple of the Apostle John and was martyred in the Roman Coliseum by being eaten by lions.
“If anyone is truly religious, he is a man of God; but if he is irreligious, he is a man of the devil, made such, not by nature, but by his own choice. …there is set before us life upon our observance [of God’s precepts], but death as the result of disobedience, and every one, according to the choice he makes, shall go to his own place, let us flee from death, and make choice of life.” Ignatius (The Epistle of Ignatius to the Magnesians Chap 5, Long Version).
“It was not until the fourth century that Gnostic and Manichaean influence started to infiltrate the Christian Church, polluting it with their doctrines. Augustine, after saturating himself in Gnostic philosophy for many years, joined the Church and became a Bishop. He then began to contradict what the Church had always taught on human nature and the freedom of man’s will and taught in accordance with the Gnostic views of human nature and free will. The Church, through the influence of Augustine, began to embrace and teach the doctrine of natural inability.
It is an undisputed and known fact of history, admitted by Augustine’s admirers and supporters in their historical accounts of his life, that Augustine was influenced by, and a member of, the Manichaean Gnostic sect. John K. Ryan, in his introduction to “The Confessions of Saint Augustine” said, “The two great intellectual influences upon Augustine prior to his conversion were Manicheism and Greek Philosophy.” (John K. Ryan The Confessions of Saint Augustine, Random House, Inc., 1960, p. 23)
In their introduction to “The Confessions of Augustine,” John Gibb and William Montgomery said, “In the same year in which he read the Scriptures and was disappointed in them, Augustine joined the Manichaean sect…” (John Gibb and William Montgomery ‘The Confessions of Augustine,’ University Press, 1908,).
They also said, “For nearly nine years Augustine was a Manichaean Auditor. At first he was a zealous partisan who contended publicly for his new faith, and did not hesitate to ridicule the doctrines of the Church and especially the Old Testament Scriptures…” (John Gibb and William Montgomery (The Confessions of Augustine, University Press, 1908, p. xxxii)