Calvinists believe in a doctrine they name total depravity, according to which they claim:
From this original corruption, whereby we are utterly indisposed, disabled, and made opposite to all good, and wholly inclined to all evil, do proceed all actual transgressions. [From the Westminster Confession of Faith, ch. VI; emphasis added]
More than one part of Scripture contradicts this erroneous doctrine. We shall be considering a number of them. First up is Psalm 106:3:
How blessed are those who keep to what is just, whose conduct is always upright! (NJB)
If the Reformed were right, the set of people identified by this verse is empty, which makes it pointless. We may as well say, “How blessed are…uh…well, never mind.” No legal fiction of mere imputation of righteousness will overcome the force of this verse. The Reformed say everyone necessarily sins; the Bible makes it clear that it is at least possible not to sin.
The Reformed are wrong on this score. They did not have to be. If they had broadened their hermeneutical horizons beyond the poetic hyperbole of a couple Psalms quoted in Romans, they might have got the right answer. Their mistake was unnecessary. If we are not free to do choose to do good or evil we cannot justly be held accountable for our sins, because we would then only be doing what is natural for us. Even worse, if it is natural for us to sin then either Jesus also had a sin nature or he was not genuinely human but rather something else similar to but different from us. This, of course, is ludicrous, and it is part of the reason why so-called total depravity is erroneous.
What they fear, evidently, is the Pelagian error that a man can merit salvation apart from grace. Their claim that the Catholic Church teaches something like this is another mistake. In point of fact even if Adam had not sinned he would still have needed grace in order to attain to heaven. Why? Because heaven is supernatural. That means (among other things) that it is above and beyond our powers entirely. We always need grace in order to see God, even if we have not sinned. To say otherwise—to suggest that if Adam had not sinned he would have merited heaven apart from grace—that is the Pelagian error. The Catholic Church has always taught otherwise, the fictions perpetuated by her adversaries notwithstanding.
[Update, 23 March 2016]
Psalm 32:11 says this:
Be glad in the LORD and rejoice, you righteous; exult, all you upright of heart.
Just who does David have in mind here when he speaks of the righteous if (as the Reformed would have us believe) no one is righteous? They can’t have it both ways; either there are righteous people whom David addresses here or there aren’t. But if there are none, then is David talking to the wind?