The Arminians get angry with the Calvinists when the latter say that God does not choose to save all men from damnation and that he might justly have left all in a state of damnation if he had so chosen. They say that this makes a cruel, mean, unloving God. In their view, it would be completely inappropriate for God to leave any in hell if he could save them. There would be something wrong with God if he did such a thing; he would not be treating his creations as they ought to be treated, as it is fitting and appropriate for them to be treated. Thus, Arminianism teaches that all men merit salvation from sin and eternal life, contrary to Scripture which teaches that all men are sinners truly deserving of hell and that salvation and eternal life are free, unmerited gifts of God's grace (Romans 3:9-26; 6:23; Ephesians 2:1-10; etc.). The great Calvinist theologian and philosopher Jonathan Edwards put the logic this way:
When men are fallen, and become sinful, God by his sovereignty has a right to determine about their redemption as he pleases. He has a right to determine whether he will redeem any or not. He might, if he had pleased, have left all to perish, or might have redeemed all. Or, he may redeem some, and leave others; and if he doth so, he may take whom he pleases, and leave whom he pleases. To suppose that all have forfeited his favor, and deserved to perish, and to suppose that he may not leave any one individual of them to perish, implies a contradiction; because it supposes that such a one has a claim to God’s favour, and is not justly liable to perish; which is contrary to the supposition. (Jonathan Edwards, “The Justice of God in the Damnation of Sinners,” in The Works of Jonathan Edwards, vol. 1 [Edinburgh: Banner of Truth Trust, 1974], 670.)
And we can go even further than this: Eternal life consists in the full and unending enjoyment of God as his adopted children, sharing in the relationship between the Father and the Son. It is something that belongs exclusively to God. No finite creature, being finite and not divine, could be naturally worthy of such a thing (Romans 8:1-30; 2 Corinthians 5:21; 1 John 3:1-2; etc.). And yet the Arminians make us to merit this infinite blessing, and thus make us by nature equal to God.
Certainly, then, Arminianism deserved the condemnation for heresy that it received at the Synod of Dordt. It is no mild error, but strikes at the heart of both the sovereignty of God and salvation by grace.
For more, see here, here, here, and here.