Kill the infidel

Muslims are infamous for beheading "apostates," "infidels," and "blasphemers"? Contemporary Christians typically denounce that practice. 

That, however, raises the question of what's wrong with executing blasphemers? After all, the church used to do that. 

Indeed, I imagine that some Muslims might be puzzled by why modern-day Christians aren't prepared to defend God's honor by punishing outspoken enemies of the faith. In traditional Catholic theology, a heretic was a soul-murderer–far worse than a physical murderer. Here's a classic statement from Aquinas:

I answer that, With regard to heretics two points must be observed: one, on their own side; the other, on the side of the Church. On their own side there is the sin, whereby they deserve not only to be separated from the Church by excommunication, but also to be severed from the world by death. For it is a much graver matter to corrupt the faith which quickens the soul, than to forge money, which supports temporal life. Wherefore if forgers of money and other evil-doers are forthwith condemned to death by the secular authority, much more reason is there for heretics, as soon as they are convicted of heresy, to be not only excommunicated but even put to death. 

On the part of the Church, however, there is mercy which looks to the conversion of the wanderer, wherefore she condemns not at once, but "after the first and second admonition," as the Apostle directs: after that, if he is yet stubborn, the Church no longer hoping for his conversion, looks to the salvation of others, by excommunicating him and separating him from the Church, and furthermore delivers him to the secular tribunal to be exterminated thereby from the world by death. For Jerome commenting on Galatians 5:9, "A little leaven," says: "Cut off the decayed flesh, expel the mangy sheep from the fold, lest the whole house, the whole paste, the whole body, the whole flock, burn, perish, rot, die. Arius was but one spark in Alexandria, but as that spark was not at once put out, the whole earth was laid waste by its flame." 

http://www.newadvent.org/summa/3011.htm#article3

So are contemporary Christians inconsistent when we refuse to execute heretics, apostates, blasphemers? 

i) Some theonomists appeal to OT blasphemy laws. One problem with that particular appeal is that ancient Israel was meant to be holy in a way that a modern nation-state is not. The purity codes and sacred spaces were designed to illustrate and protect Israel's cultic holiness. That, in turn, was an emblem of God's holy presence among an unholy people. Hence, these symbolic buffers were put in place. 

As a result, certain behavior was punishable that would not otherwise be punishable. There is, however, little evidence that this carries over into the new covenant. 

Of course, blasphemy is still a sin. That's part of the moral law, and not merely the ceremonial law. But some behavior was punishable as a result of Israel's heightened holiness that would not be punishable absent that framework of cultic holiness. 

Complementing Israel's unique status were certain safeguards. There were supernatural methods of ascertaining guilt or innocence (e.g. trial by ordeal, the Urim and Thummin). God sent prophets. Sometimes God directly intervened when things got out of hand. 

A modern-nation state lacks these safeguards. 

ii) Conversely, some Christians say blasphemy laws run counter to the Sermon on the Mount. We should love our enemies. 

However, Aquinas would counter we are commanded to love our neighbors, which requires us to protect them from soul-murder. Which brings us to the next point:

iii) God cannot be harmed. So he doesn't need us to protect him. God is not analogous to a murder victim (pace Aquinas). As Scripture says, in another context:

 But Joash said to all who stood against him, “Will you contend for Baal? Or will you save him? Whoever contends for him shall be put to death by morning. If he is a god, let him contend for himself, because his altar has been broken down”(Judges 6:31).

iv) Of course, Aquinas would counter that this misses the point. It's not God who is harmed. Rather, the heretic is murdering the souls of innocent human victims. It's our duty to protect them from heretics.

That, however, is a very slack analogy. As a rule, murder victims don't consent to their own murder. By contrast, people who believe heresy are willing victims. They are not innocent or involuntary parties to that transaction. 

We have limited responsibility for what someone chooses to do with his own life. That's ultimately between him and God. God will judge him. 

For instance, there are people who spend their money in wasteful, foolish, or sinful ways. But as a rule, how they spend their money is none of my business. They don't answer to me. I don't have the right to boss them around.

v) Apropos (iv), just because somebody does something wrong doesn't ipso facto mean I have the right to punish him. If I see a poker player cheating at cards, it's not my duty to avenge his wrongdoing. 

vi) Another glaring problem with blasphemy laws is that it empowers the state to define true religion. That, in turn, gives the state a pretext to persecute the faithful. A statute designed to punish heresy can become its antithesis if government officials are heretics or atheists. They will simply redefine the target to suit their agenda.

vii) God himself will punish impenitent heretics, blasphemers, et al. It's not as if they get off Scott free. The duty of the state is to keep criminality at manageable levels so that the righteous can survive and flourish. It's not the duty of the state of dispense eschatological justice.