One problem with Calvinism is that, whereas Romans 5 strongly indicates that the gift is greater than the offence, Calvinism would require that the offence be greater than the gift.
When Adam sinned, his sin spread to all of his descendants. Paul tells us this much: "As through one trespass there is condemnation for everyone, so also through one righteous act (the death of Christ) there is life-giving justification for everyone."
But clearly not everyone is saved. Many people will be condemned, and indeed already have been. So how do we reconcile this? Let us back up a little to verse 12. "Therefore, just as through one man sin entered the world, and death through sin, and thus death spread to all men, because all sinned...". Is it possible, then, to avoid Adam's trespass? Absolutely! If a man could be born in Adam's line, and yet sin at no point during his life, he would not be condemned for Adam's sin.
As it happens, such a man did once walk the earth. Jesus, though He was descended from Adam, was not condemned because of Adam's sin. At least, not in the sense that He deserved it. The only reason Jesus died for Adam's sin is that Jesus died to redeem those who were cursed on Adam's account. If anyone else lived so perfect a life as Jesus, they too would avoid condemnation.
But of course, no one else ever has, nor indeed ever will. Everyone died because everyone sins. The same is true of the gift. Just as everyone could avoid condemnation by living a sinless life from conception to death, everyone could miss out on the gift of salvation by disbelief. It's not that the cross doesn't account for everyone, but that many people won't allow it to account for them. As death spread to all men because all sin, so also can life spread to all men if all believe. Therefore, only if we say that the offense is greater than the gift can we say that Limited Atonement is a valid doctrine.