Here is St Augustine in AD 428/429 (1-2 years before his death in AD 430) said
But, lest perchance it be said that perseverance even to the end is not indeed lost when it has once been given—that is, when a man has persevered unto the end—but that it is lost, in some sense, when a man by contumacy so acts that he is not able to attain to it; just as we say that a man who has not persevered unto the end has lost eternal life or the kingdom of God, not because he had already received and actually had it, but because he would have received and had it if he had persevered; — let us lay aside controversies of words, and say that some things even which are not possessed, but are hoped to be possessed, may be lost.--St Augustine, On the Predestination of the Saints, Book II: On the Gift of Perseverance, Chapter 11
Let the inquirer still go on, and say, "Why is it that to some who have in good faith worshipped Him He has not given to persevere to the end?" Why except because he does not speak falsely who says, "They went out from us, but they were not of us; for if they had been of us, doubtless they would have continued with us." [1 John 2:19] Are there, then, two natures of men? By no means. If there were two natures there would not be any grace, for there would be given a gratuitous deliverance to none if it were paid as a debt to nature. But it seems to men that all who appear good believers ought to receive perseverance to the end. But God has judged it to be better to mingle some who would not persevere with a certain number of His saints, so that those for whom security from temptation in this life is not desirable may not be secure. For that which the apostle says, checks many from mischievous elation: "Wherefore let him who seems to stand take heed lest he fall." [1 Corinthians 10:12] But he who falls, falls by his own will, and he who stands, stands by God's will. "For God is able to make him stand;" [Romans 14:4] therefore he is not able to make himself stand, but God. Nevertheless, it is good not to be high-minded, but to fear. Moreover, it is in his own thought that every one either falls or stands. Now, as the apostle says, and as I have mentioned in my former treatise, "We are not sufficient to think anything of ourselves, but our sufficiency is of God." [2 Corinthians 3:5] Following whom also the blessed Ambrose ventures to say, "For our heart is not in our own power, nor are our thoughts." And this everybody who is humbly and truly pious feels to be most true.--St Augustine, On the Predestination of the Saints, Book II: On the Gift of Perseverance, Chapter 12What else is interesting about this treatise is St Augustine uses many of the popular Calvinist proof text without coming to the Calvinist conclusions, and uses 1 John 2:19 and interprets it to refer to the elect and non elect, whereas Calvinists generally read it as false converts and true Christians.
The following is St Augustine in AD 427, 3 years before his death:
It is, indeed, to be wondered at, and greatly to be wondered at, that to some of His own children— whom He has regenerated in Christ— to whom He has given faith, hope, and love, God does not give perseverance also, when to children of another He forgives such wickedness, and, by the bestowal of His grace, makes them His own children. Who would not wonder at this? Who would not be exceedingly astonished at this?--St Augustine, On Rebuke and Grace, Chapter 19This is perhaps the most explicit denial of Calvinism in the later writings of St Augustine, saying some are "REGENERATED" and "His own children" yet NOT GIVEN PERSERVERANCE.
Here is St Augustine in AD 427 (3 years before his death in 430) writing to Boniface, a man whom St Augustine fears may have or may lose salvation by sins:
What shall I say regarding these evils— so patent to all, and so great in magnitude as well as number— of which you have been, directly or indirectly, the cause since the time of your being married? You are a Christian, you have a conscience, you fear God; consider, then, for yourself some things which I prefer to leave unsaid, and you will find for how great evils you ought to do penance; and I believe that it is to afford you an opportunity of doing this in the way in which it ought to be done, that the Lord is now sparing you and delivering you from all dangers.--St Augustine, (AD 427) Letter 220 Paragraph 5This paragraph 5 where St Augustine states to Boniface, he is a Christian, so any Calvinist ideas of tares, cannot be read in.
Nevertheless, I would not wish you to belong to the number of those wicked and unjust men whom God uses as instruments in inflicting temporal punishments on whom He pleases; for He who justly employs their malice to inflict temporal judgments on others, reserves eternal punishments for the unjust themselves if they be not reformed. Be it yours to fix your thoughts on God, and to look to Christ, who has conferred on you so great blessings and endured for you so great sufferings.--Letter 220 Paragraph 8St Augustine here expresses his hope Boniface does not became one of the unjust that are hellbound.
But if you consult me regarding your relation to God and the salvation of your soul, and if you fear the word of truth which says: "What is a man profited, if he shall gain the whole world, and lose his own soul?" [Matthew 16:26] I have a plain answer to give. I am prepared with advice to which you may well give heed. But what need is there for my saying anything else than what I have already said. "Love not the world, neither the things, that are in the world. If any man love the world, the love of the Father is not in him. For all that is in the world, the lust of the flesh, and the lust of the eyes, and the pride of life, is not of the Father, but is of the world. And the world passes away, and the lust thereof: but he that does the will of God abides forever." [1 John 2:15-17] Here is counsel! Seize it and act on it. Show that you are a brave man. Vanquish the desires with which the world is loved. Do penance for the evils of your past life, when, vanquished by your passions, you were drawn away by sinful desires. If you receive this counsel, and hold it fast, and act on it, you will both attain to those blessings which are certain, and occupy yourself in the midst of these uncertain things without forfeiting the salvation of your soul.--Letter 220 Paragraph 9
St Augustine plainly warned him not to forfeit salvation by loving the things of the world!
Finally, lets see St Augustine's view of works and salvation--had a modern Catholic stated this a Calvinist or Baptist would scream "works based!":
For who is so ignorant, who so foolish, as not to see that the health of this mortal body, and the strength of its corruptible members, and victory over men who are our enemies, and temporal honours and power, and all other mere earthly advantages are given both to the good and to the bad, and are taken away both from the good and from the bad alike? But the salvation of the soul, along with immortality of the body, and the power of righteousness, and victory over hostile passions, and glory, and honour, and everlasting peace, are not given except to the good. Therefore love these things, covet these things, and seek them by every means in your power. With a view to acquire and retain these things, give alms, pour forth prayers, practise fasting as far as you can without injury to your body. But do not love these earthly goods, how much soever they may abound to you. So use them as to do many good things by them, but not one evil thing for their sake. For all such things will perish; but good works, yea, even those good works which are performed by means of the perishable good things of this world, shall never perish.--Letter 220 Paragraph 11Here St Augustine states "retain these things," referring to the previous sentence, which included salvation. Calvinism has no place for telling someone to "retain" salvation. Also, notice St Augustine claims these things are given ONLY to the "good."
Certainly, there is much more that can be said about St Augustine and his view of salvation. This should suffice to show St Augustine did not magically convert a Calvinistic view of perserverance of the saints in his later life.