Well, the Bible says it isn’t. It’s the well-meaning Arminian who wants to take the promises of God and apply them to “all people, in all places, for all time” but they forget that God is a God of justice and wrath against sin:
The LORD passed before him and proclaimed, “The LORD, the LORD, a God merciful and gracious, slow to anger, and abounding in steadfast love and faithfulness, keeping steadfast love for thousands, forgiving iniquity and transgression and sin, but who will by no means clear the guilty, visiting the iniquity of the fathers on the children and the children’s children, to the third and the fourth generation.”~ Exodus 34:6–7 (ESV)
God cannot allow people to go unpunished for their sins. Follow me through Romans 1 here:
For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who by their unrighteousness suppress the truth. For what can be known about God is plain to them, because God has shown it to them. For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made. So they are without excuse.~ Romans 1:18–20 (ESV)
1) “The wrath of God is (Rom 2:5) revealed from heaven against *all* ungodliness and unrighteousness of men.” As Jesus makes clear in his sermon on the mount (Mt 5-7), God sees all, including our thought life, and holds us accountable for all of it, treating even our secret desires as if we’ve actually physically committed the sins.
2) “For what can be known about God is plain to them, because God has shown it to them.” How has God revealed it to them?
For when Gentiles, who do not have the law, by nature do what the law requires, they are a law to themselves, even though they do not have the law. They show that the work of the law is written on their hearts, while their conscience also bears witness, and their conflicting thoughts accuse or even excuse them~ Romans 2:14–15 (ESV)
Yet he did not leave himself without witness, for he did good by giving you rains from heaven and fruitful seasons, satisfying your hearts with food and gladness.”~ Acts 14:17 (ESV)
The God who made the world and everything in it, being Lord of heaven and earth, does not live in temples made by man, nor is he served by human hands, as though he needed anything, since he himself gives to all mankind life and breath and everything. And he made from one man every nation of mankind to live on all the face of the earth, having determined allotted periods and the boundaries of their dwelling place, that they should seek God, and perhaps feel their way toward him and find him. Yet he is actually not far from each one of us,~ Acts 17:24–27 (ESV)
3) Expanding further on this, in Rom 1:20 Paul states, “For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made.” That “have been” is a damning statement. The only way it can be understood is that God, in order to justify himself and his laws before men, presses upon the hearts of mankind his law. We call that the “conscience”. The unregenerate conscience is only capable of revealing God’s standard to men but their actions against this conscience will harden them against it (Deut 2:30; 1 Sam 6:6; Ps 95:8; Mark 8:17; Heb 3:8, 15; 4:7) the same way that God hardened the heart of Pharoah and Sihon, King of Heshbon, against the Israelites (Exo 4:21; Deut 2:30). God also will harden his heart, even against his own people, if they live in rebellion against him (Isa 63:17).
4) “So they are without excuse.” There is no deeper fear that man can have than to be at enmity with God, and the unregenerate man lives in this state. But they sear their consciences through continued transgression against God’s holy standard bit-by-bit, day-by-day until it becomes routine. Eventually they battle against God, demanding that God bend to their will.
Now, a quick-witted arminian will look at Romans 1:18 and say “does ‘ALL’ mean ‘ALL”‘, but to deny that God is sovereign over all of creation means that he’s not God – merely able to create and rule over some of his creation, but a weak entity who cannot help those whom he chooses to help. And we know from scripture that God chooses people and nations for his own purpose (Gen 18:19; 1 Sam 20:30; 2 Sam 16:18; 1 Ki 8:44, 48; 11:13, 32, 36; 2 Ki 21:7; 23:27; 1 Chr 28:6; 2 Chr 7:16, 33:7; Neh 1:9; Ps 89:3, 19; Isa 41:8-9; 42:1; 43:10; 44:1-2; Hag 2:23; Mt 12:18; John 13:18).
If not, and if this applies to all people, then all people need hearts of flesh, and not of stone. Yet we don’t see that in scripture. We only see the people whom God has saved receiving the heart of flesh:
And I will give them one heart, and a new spirit I will put within them. I will remove the heart of stone from their flesh and give them a heart of flesh, that they may walk in my statutes and keep my rules and obey them. And they shall be my people, and I will be their God. But as for those whose heart goes after their detestable things and their abominations, I will bring their deeds upon their own heads, declares the Lord GOD.”~ Ezekiel 11:19–21 (ESV)
And I will give you a new heart, and a new spirit I will put within you. And I will remove the heart of stone from your flesh and give you a heart of flesh. And I will put my Spirit within you, and cause you to walk in my statutes and be careful to obey my rules.~ Ezekiel 36:26–27 (ESV)
For this is the covenant that I will make with the house of Israel after those days, declares the LORD: I will put my law within them, and I will write it on their hearts. And I will be their God, and they shall be my people.~ Jeremiah 31:33 (ESV)
So, do we see this “heart of flesh” in all people? No. instead we see what is shown above – that men live unrepentant lives, constantly searing their conscience against God’s law, and continually bearing the weight of their own sin debt before God. That sin debt has to be taken by someone. Either they take the punishment on their own and spend eternity in Hell, or they place their trust in Jesus, in the sufficiency of the work on the cross to pay their sin debt, in the efficacy of the work on that cross to apply to their lives, and in the promises that God has made them into a new creation (2 Cor 5:17), dead to sin and alive to God in Christ Jesus (Rom 6:11).
Now the offer of salvation is freely given to all men, but God prevents some from receiving it. The well-meaning Arminian and Atheist alike will look at the decrees of God that those whom God has provided the faith necessary for conversion (Jn 3:6, 6:63; Acts 5:31, 11:18, 16:14; Rom 11:36, 12:3; Eph 2:8-9; 1 Cor 4:7, 12:3; 2 Pet 1:3) and say that it’s “unfair” for God to only choose some and not all, but as Romans 9 clearly states, who are we (the created beings) to call out to him who made us and demand that he follow our desires?
What if God, desiring to show his wrath and to make known his power, has endured with much patience vessels of wrath prepared for destruction, in order to make known the riches of his glory for vessels of mercy, which he has prepared beforehand for glory—even us whom he has called, not from the Jews only but also from the Gentiles?~ Romans 9:22–24 (ESV)
So, of the countless millions upon millions running headlong into hell, gladly searing their consciences as they pursue their unholy proclivities, God, by his own will and to his own glory, chooses some from that throng of unbelievers, invades their lives, replaces their heart so that they can hear his word, then interferes with them to change their minds. In some cases that interference is merely the word of some street preacher or evangelist, but most often it’s seen through countless minor changes in their conscience – now reawakened – which lead them to see the world a new, and eventually they see their sin as what it is before God and they can do no other than to drop to their knees in repentance and faith in the God who died in their place to save them.
This is how a Calvinist can see the “whoever” of John 3:16 as what it is – “πᾶς ὁ πιστεύων” “All the believing”. God provides his offer of salvation “freely” to all that have ever lived, but he has no intention to save all so that his communicable attributes of wrath and justice would be revealed upon mankind. Instead, as is shown in Romans 9 above, God reveals endures through his communicable attributes of long-suffering and mercy the “vessels of wrath” (humans intended for judgement and damnation) in order that his mercy and grace would shine all the more brightly when he saves those whom he has desired to save. God has no obligation to save anyone as all have sinned against him (Ps 14:1-3; 53:1-3; Jer 17:9; Rom 3:2, 23) and violated his perfect standard:
You therefore must be perfect, as your heavenly Father is perfect.~ Matthew 5:48 (ESV)
So, going back to the original point, yes, God’s offer of salvation is given to all, but only those whom he has elected for salvation are able to turn and repent.
Unless it’s your assumption that Jesus’ work on the cross has been applied to all, and is sufficient and effective for all people, in every time and place, then that means that God has doubly punished for sin – once upon Jesus and a second time upon the individual who dies in their sins. This makes God unjust as Jesus took more punishment than was necessary in his humiliation, condemnation, and suffering on the cross, then still punishing people for the sins that Jesus took on their behalf. This is heresy.