When it comes to the assurance of salvation, no philosophy is stronger than Calvinism. Calvinism teaches that although man is totally depraved, and thus cannot come to Christ of his own free will, God unconditionally elects a limited number of people, who will be compelled to come to Christ, being incapable of permanently falling away.
Although I cannot affirm all of Calvinism's 5 points, I am so firmly entrenched in the Perseverance of the Saints, a.k.a. Eternal Security, that I almost see the denial thereof as apostasy. If you do not affirm Eternal Security, first of all, I believe you are denying Scripture, but I also believe you are sitting so dangerously close to professing a works-based gospel that I fear for your precious soul.
Eternal Security is often oversimplified as "once saved, always saved". To me, this oversimplification is incredibly annoying. Technically, it's accurate. Once a man has received salvation, Scripture says God seals us with His Holy Spirit as a guarantee for the day of redemption (2 Corinthians 1:22; 5:5; Ephesians 1:13-14; 4:30), and that, having begun His work on us, He will see it through to completion (Philippians 1:6). It even tells us that those who depart from the faith were not truly of it, for if they were, they would have remained in it, but they left in order that it may be shown that they were never truly of it (1 John 2:19). In other words, yes, once you are saved, you are always saved.
But as early as the mid 50s A.D., dissenting voices were already criticising the Gospel, saying "then let us continue to sin that grace may abound". To this very day, heretics of all shapes and sizes continue to assert that if salvation is so easily secured, we may as well go on living in sin. As a non-Calvinist, this annoys me to no end, but against a Calvinist, we have the additional problem of election; we may as well continue to sin, since God may either will to condemn you regardless, and thus you have no path to salvation, or to save you regardless, and thus you are in no danger of condemnation.
I stand arm in arm with my Calvinist brethren in condemning these petty caricatures. But Calvinists have their own oversimplifications of other views. Take, for example, "God cannot save me without my consent". Calvinists argue, in spite of explicit Scriptures to the contrary, that it is not God's desire for all men to be saved and come to the knowledge of the truth. This, of course, would conflict with Calvinism in numerous ways. It is contrary to Unconditional Election, as it implies there is some way in which God distinguishes between those who will be saved, as He desires, and who will not be, in spite of His desire. It is contrary to Limited Atonement, as it would at least seem to imply that the Gospel is open to those who will reject it. And of course, if even one person resists God's grace, the idea of irresistible grace is thoroughly debunked.
But Calvinists depend on God being entirely sovereign over who will be saved (which, I will add, is common ground for us). If God cannot do something, this is seen as a detraction from His sovereignty. If "God cannot save me without my consent", then God is, in fact, not sovereign.
The oversimplification that "God cannot save me without my consent", however, is actually worse than "once saved, always saved". Why? Because "once saved, always saved", while oversimplified, is at least technically accurate. "God cannot save me without my consent", however, is not.
See, first, God can do whatever He pleases. But first of all, there are exceptions. Exceptions which are explicit in Scripture. God "...cannot lie..." (Titus 1:2), and "...cannot deny Himself" (2 Timothy 2:13). These two explicit Biblical statements make contradictions impossible for God. He cannot make a square triangle, a married bachelor, or... a compelled free choice.
Scripture also tells us very plainly that "...without faith it is impossible to please Him, for he who comes to God must believe that He is, and that He is a rewarder of those who diligently seek Him." (Hebrews 11:6). This verse is loaded with assumptions. First, that it is actually possible to diligently seek God. Second, that it is God's nature to reward those who diligently seek Him. Third, He cannot be pleased with those who do not. So, can God save an unbeliever? Logically, it does not appear so.
But is God unable to make an unbeliever believe? Well, actually, no. In fact, a time is coming in which every tongue will confess that Jesus Christ is Lord (Romans 14:11). This, of course, will come through sight. Scripture seems to indicate that the closer an encounter with God, the more firmly we affirm Him. Time and time again, in Scripture, those who encounter God closely recognise Him by instinct. In Job 42:5-6, Job declares "“I have heard of You by the hearing of the ear, But now my eye sees You. Therefore I abhor myself, And repent in dust and ashes.”" Isaiah, encountering God, likewise cried out "“Woe is me, for I am undone! Because I am a man of unclean lips, And I dwell in the midst of a people of unclean lips; For my eyes have seen the King, The Lord of hosts.”" (Isaiah 6:5). Even Paul, being collared by the Lord, asked Him "...Who are You, Lord?..." (Acts 9:5), of course leading to His repentance. His testimony to Timothy reveals that his prior life was out of ignorance (1 Timothy 1:13).
So, can God save a man without his consent? Absolutely, simply because there is no conflict between a free choice and an influenced choice. God, of course, knows every possible outcome of every possible scenario, and has control over scenarios we do not. God knows every set of circumstances that would influence a person to freely choose salvation. This is what Molinists would call "middle knowledge".
No Calvinist can afford to deny middle knowledge. Not only are we told that God knows all things (e.g. 1 John 3:20), but we even see examples of God using that knowledge. In His condemnations of Charozin, Bethsaida, and Capernaum, Jesus tells them that if He had done His mighty works in Tyre, Sidon, and Bethsaida, these cities would have repented (Matthew 11:20-24; Luke 10:13-16).
Of course, this passage also shows God's sovereignty in salvation, simply because although these other cities would have repented, God denied them that chance. But supposing God had spared them without doing the works. Would they have repented anyway? In truth, only God knows. We do not possess middle knowledge, nor does Scripture say whether they would have repented. Nevertheless, what it does say is they would have repented if Christ had done His mighty works in these cities, and that this is why the impenitent cities will find Judgement Day less tolerable than they would.
With all of that in mind, it is woefully inadequate to say that God cannot save those who do not consent. A more accurate statement is God will not save those who do not consent. Of course, even this is inadequate, since Scripture says God takes no pleasure in the death of the wicked (Ezekiel 18:32; 33:11), and wants no one to perish (2 Peter 3:9). Thus, someone who does not consent at one point in time may later gratefully receive the gift of salvation at a later time. According to His own grace and wisdom, God may then choose whether to grant further grace, or, as is His prerogative, withdraw grace and condemn them.
In the end, God remains in full control. There are some things He cannot do, and that does include contradiction. Thus, as faith is the condition for salvation, He cannot save those who do not have faith. Furthermore, as He has granted us free will, He cannot logically compel us to have faith. However, it is quite within His power to save anyone and everyone. Therefore, He receives all credit for salvation, and no blame for sin deserving of condemnation. When someone is condemned, it is not because God could not save them, but also not because God did not will for them to be saved; His word explicitly tells us He does. But it also tells us that we can reject the gift of salvation He so gladly offers, and that we, not He, lose out in that scenario. This is the beauty of grace; the threat of justice is very real, making mercy far more glorious when offered than if it was thrust upon all, with no choice involved. Thus, we see that God absolutely can save even the most hard hearted individual, but for His own reasons, He is quite willing to respect the will of those vessels who choose to remain objects of His wrath, as each and every one of us once were (Ephesians 2:1-3).