Some advance the premise that the Atonement of Christ is unlimited... mostly. This is the starting point for most of the confusion you're dealing with.
That is to say, they believe all sin has been atoned for, and there is no more judgment due for it. So, what still condemns people? Supposedly, not taking advantage of the offer of mercy, "rejecting the Son."
But, isn't this response SIN? If so, then it too has been atoned for, yes? If it hasn't, then everyone has one (infinite) sin to pay for themselves, correct? In which case, there's no hope for anyone, because no one can cover an infinite debt.
Or is it only some people (believers) who have this sin paid for, and some (the rejecters) don't? Who calls that shot? Who is ultimately responsible for salvation in that case? And if this situation is true, is it really honest to say that ALL sin has been paid for? This is telling people, "Everything (almost) has been done for you... there's just this one little thing you have to do to contribute to your salvation... without which you won't get your gift... you get the last little bit forgiven IF you do this... just believe..." And for some, "don't stop believing, or it's all for naught."
Some folks try to evade the dilemma by asserting that rejecting God's offer isn't sin. But refusal is the height of unbelief; it constitutes a positive claim that God's Word is open to legitimate doubt, and that calling his veracity in question isn't blameworthy. This theory denies (at the deepest level) that men have a duty to believe God. (If it isn't a duty, then it's "optional.") Opting for hell isn't sin? Preference for separation from God, for "happiness" or "good" without him? Sure it is. It was the very first sin.
For others, their preferred solution is either to admit universalism (that everyone must be saved), or to protest the demand for consistency. The latter would just rather live with the various difficulties their a prioris have forced upon them.
Thus, the basic question comes back to the matter of whether, for every person considered individually, ALL his sin paid for, or NONE of it? Even if it was just one sin unpaid for, it might as well be all of them since the result would be the same. Scripture texts have already been set forth above, clearly stating that all manner of sin brings one into judgment.
