In Calvin's commentary on 1st John 3:9, he uses the word "perseverance", clearly stating:
"the hearts of the godly are so effectually governed by the Spirit of God, that through an inflexible disposition they follow his guidance." (John Calvin)
"the power of the Spirit is so effectual, that it necessarily retains us in continual obedience to righteousness." (John Calvin)
This is NOT eternal security. Rather Perseverance of the Saints is a perseverance in a behavior consistent with that expected of a child of God. However, Calvin failed to accept the FACT that believers DO sin, sometimes horribly so.
That's what modern Calvinist missunderstood, and luckily they are right on preservation, without knowing they disproofed the heresy of Calvinism.
Perseverance of the saints and eternal security are definitely not the same. Perseverance of the saints as Calvin taught does not allow the saint to know whether he is chosen or not.
Psalm 91 King James Version (KJV)
91 He that dwelleth in the secret place of the most High shall abide under the shadow of the Almighty.
2 I will say of the Lord, He is my refuge and my fortress: my God; in him will I trust.
3 Surely he shall deliver thee from the snare of the fowler, and from the noisome pestilence.
4 He shall cover thee with his feathers, and under his wings shalt thou trust: his truth shall be thy shield and buckler.
5 Thou shalt not be afraid for the terror by night; nor for the arrow that flieth by day;
6 Nor for the pestilence that walketh in darkness; nor for the destruction that wasteth at noonday.
7 A thousand shall fall at thy side, and ten thousand at thy right hand; but it shall not come nigh thee.
8 Only with thine eyes shalt thou behold and see the reward of the wicked.
9 Because thou hast made the Lord, which is my refuge, even the most High, thy habitation;
10 There shall no evil befall thee, neither shall any plague come nigh thy dwelling.
11 For he shall give his angels charge over thee, to keep thee in all thy ways.
12 They shall bear thee up in their hands, lest thou dash thy foot against a stone.
13 Thou shalt tread upon the lion and adder: the young lion and the dragon shalt thou trample under feet.
14 Because he hath set his love upon me, therefore will I deliver him: I will set him on high, because he hath known my name.
15 He shall call upon me, and I will answer him: I will be with him in trouble; I will deliver him, and honour him.
16 With long life will I satisfy him, and shew him my salvation.
"the hearts of the godly are so effectually governed by the Spirit of God, that through an inflexible disposition they follow his guidance." (John Calvin)
"the power of the Spirit is so effectual, that it necessarily retains us in continual obedience to righteousness." (John Calvin)
I believe this is what John Calvin saw! It is by "traditions of men", that have rendered "void" any, and/or ALL references to every "Saint of God" (see parable of the prodigal son) , save for the only "begotton!"
For, you see, "In the Spirit" of God? this "preservation of the saints", is maintained BY God Himself, and IS "passed on" "through", and TO ones' "born in flesh!" These ones, seen in this fallen world, and through "flesh eyes", as "different!" They don't "fit!" "They DON'T get it!" "ANTI-social", "touched", "autistic", "backwards", of the which, in EVERY case, is merely God's ATTEMPTINGS, in His "Preservationing" OF His Saints!" "With psychiatric help, and "meds?", "They'll soon be a more productive member of our assembling, and erecting this beast, as we rise it from the bottomless pit!" THIS, is the WHERE, and WHAT, the "
PERSERVERENCE of the Saints" means, and comes into play!
You see, the enemies of the Kingdom of God, and Kingdom of Light, are so "adept" at "rendering" one "born of waters'" spirit, as to make them "virtually USELESS" in Aiding Our Heavenly Father, in His "exectuing" Psalms-110:1b!
(You also stated
"However, Calvin failed to accept the FACT that believers DO sin, sometimes horribly so."
However, it is of my opinion, that Calvin, for whatever reason/s, DIDN'T, or, couldn't "come to grips" that it was God Himself, that "allowed sin" into a life, for the "sole reason/s", of "SHEPHERDING", or, "cutting out" FROM (tis a prison term, but it fits) the "GENERAL POPULATION!"
So? If you are of a mind set that the "other son" in the parable of the Prodigal son. was just there, to provide "contrast?" And, that
ONLY 1/3rd of the "stars of heaven" were swept away by the "dragon's tale?" Meaning, the enemy is
vastly outnumbered? The
question HAS to be asked! Just WHAT, are we
missing? Or?
WHERE are we ALL at? ? 
