One of many scriptures clearly promoting the doctrine of Eternal Security is 1 John 5:13, which tells us "These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God." Here, John is concluding his epistle by telling us its purpose, namely to tell us that if we believe on the name of the Son of God, we can be entirely assured of our own salvation.
This makes perfect sense. Realistically, there could only be two possible ways to lose salvation. The first is to lose faith. The problem with this is that this same John tells us "They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us." (1 John 2:19). The second way to lose salvation is through sin. Yet, because salvation is by faith and not of works, this is illogical.
Not only is it illogical to say that sin loses us salvation if it is obtained by faith, it is likewise illogical to say that if we can lose salvation, we can know that we have eternal life. To verify this, let's ask one simple question: Are you saved? According to John, if we believe on the name of the Son of God, the answer is yes. I believe on the name of the Son of God, therefore I know that I have eternal life.
By contrast, a believer in Conditional Security cannot answer in this way. There is one question these people can give only one solid answer to: How do you lose salvation? They can say through apostasy. After all, you do have to endure to the end. The only way to block this interpretation is by pointing out that those who do not endure to the end never had salvation to lose in the first place. But beyond this answer, it is impossible to say which sins lose a man his salvation. Therefore, a believer in Conditional Security is forced to either downplay his own sin, or admit that, since he is a sinner, he can never truly know if he's in a state of "grace" (though grace, if it is sustained by works, is no longer grace).
Eternal Security is the only realistic interpretation of 1 John 5:13. A believer in Conditional Security is forced to either say that he knows salvation is achievable, but not if he has achieved it, or that he knows he has life, but it is not yet eternal. Only a believer in Eternal Security can truly say "because I believe on the name of the Son of God, I know I have eternal life". Therefore, the only way to sustain Conditional Security is to expunge 1 John 5:13 from scripture entirely.