The real cruncher for the doctrine of eternal security and Calvinism in general, is that historical writings show that the early church did not believe in it. While we can't use early writings for doctrine, only the bible for that, we do have historical records of what the early church believed. It should be fairly clear that they were not Calvinist:
Here's a summary of what they believed before 500 AD:
a) re: eternal security:
120-205 AD Irenaeus "Christ will not die again on behalf of those who now commit sin because death shall no more have dominion over Him.... Therefore we should not be puffed up.... But we should beware lest somehow, after [we have come to] the knowledge of Christ, if we do things displeasing to God, we obtain no further forgiveness of sins but rather be shut out from His kingdom" (Heb. 6:4­6). (Irenaeus, pupil of Polycarp, Against Heresies bk. 4, chap. 27, sec. 2)
b) re: Free will:
260-315 AD Methodius "Those [pagans] who decide that man does not have free will, but say that he is governed by the unavoidable necessities of fate, are guilty of impiety toward God Himself, making Him out to be the cause and author of human evils. " (Methodius The Banquet of the Ten Virgins discourse 8, chap. 16)
190 AD Clement of Alexandria "Neither praise nor condemnation, neither rewards nor punishments, are right if the soul does not have the power of choice and avoidance, if evil is involuntary." (Clement Miscellanies bk. 1, chap. 17)
c) re: "faith only":
30-100 AD Clement of Rome, who was a, wrote, "It is necessary, therefore, that we be prompt in the practice of good works. For He forewarns us, 'Behold, the Lord comes and His reward is before His face, to render to every man according to his work.' ... Let us therefore earnestly strive to be found in the number of those who wait for Him, in order that we may share in His promised reward. But how, beloved ones, shall we do this? By fixing our thoughts on God by faith. By earnestly seeking the things that are pleasing and acceptable to Him. By doing the things that are in harmony with His blameless will. And by following the way of truth, casting away from us all unrighteousness and sin." (Clement of Rome Letter to the Corinthians chaps. 34, 35 [companion of the apostle Paul and overseer of the church in Rome])
69-156 AD Polycarp "He who raised Him up from the dead will also raise us up-if we do His will and walk in His commandments and love what He loved, keeping ourselves from all unrighteousness." (Polycarp, personal companion of the apostle John, Letter to the Philippians chap. 2)
So Calvinists are forced to either
a) prove that so close to the time of the apostles, the church had already and quickly fallen into false doctrines,
or
b) admit that the modern doctrine of eternal security is a heretical doctrine, as it was not taught by Christ, the apostles, or in the early church.