Let me clarify myself: Augustine believed that the Elect cannot lose their salvation. While he gave warnings not to fall away, he did not actually believe the Elect could do so. He believed that the non-elect could, for a time, taste of the gifts of grace, either through Baptism or by actual conversion for a time, and then fall away. These could lose their salvation, since God would not give them the grace to persevere:
“But of such as these [the Elect] none perishes, because of all that the Father has given Him, He will lose none. John 6:39 Whoever, therefore, is of these does not perish at all; nor was any who perishes ever of these. For which reason it is said, They went out from among us, but they were not of us; for if they had been of us, they would certainly have continued with us. John 2:19”. (Augustine, Treatise on the Predestination of the Saints)
"... the human will does not obtain grace by freedom, but obtains freedom by grace; when the feeling of delight has been imparted through. the same grace, the human will is formed to endure; it is strengthened with unconquerable fortitude; controlled by grace, it never will perish, but, if grace forsake it, it will straightway fall; by the Lord's free mercy it is converted to good, and once converted it perseveres in good; the direction of the human will toward good, and after direction its continuation in good, depend solely upon God's will, not upon any merit of man. Thus there is left to man such free will, if we please so to call it, as he elsewhere describes: that except through grace the will can neither be converted to God nor abide in God; and whatever it can do it is able to do only through grace. "(Augustine, Aurelius. Augustine's Writings on Grace and Free WIll (Kindle Locations 45-46). Monergism Books. Kindle Edition.)
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