Asserting a canonical defense is one thing; proving it another
Riccitelli's case might not be an ideal one through which to encourage, even on a small scale, civil law to recognize, where appropriate, the relevance of canon law in American law and society, but it's preferable, I think, to its serving as an occasion to repudiate entirely civil consideration of canonical arguments. With all the usual caveats about commenting on stories reported in the secular press, let's see why.
Defendants in state criminal cases sometimes raise what are called "affirmative defenses", that is, arguments that their conduct, though apparently in violation of criminal law, was justified by something not mentioned in the law. A common example would be that a speeding ticket should be dismissed because a father was rushing his pregnant wife to the hospital. There is no "pregnant wife" exception in the motor vehicle code, but a defendant would probably be allowed to offer those facts as an affirmative defense (here, "necessity") to avoid a speeding conviction. A judge/jury might or might not accept the argument (the burden usually lies on the defendant to prove affirmative defenses), but generally a defendant is allowed to raise it.
Similarly, an officer of XYZ Corp who makes expenditures that appear to be in violation of, say, state laws about fiduciary duties to the corporation, could argue that, under the by-laws of XYZ corporation, such odd expenditures were in fact permitted. Again, he might fail to prove his claim, but it's a plausible defense and generally should be heard. So in this case, Riccitelli is claiming that canon law authorized him as then-pastor to spend parish money in ways that appear to prosecutors (and to the Diocese of Phoenix, for that matter) to be criminal.
It seems to me that Riccitelli should be allowed to raise canon law as a possible defense, for indeed, he was supposed to be administering parish property in accord with canon law (1983 CIC 532). For a judge to allow a canonical affirmative defense is clearly not tantamount to opening the door to prosecuting people for violating canon law or sharia or anything else; Riccitelli is being charged with violating Arizona law, not canon law. The judge, Hon. Silvia Arellano, seems to understand this, as do the appellate courts of Arizona.
At the same time, though, if the facts alleged in the media are borne out, prosecutor Barnett Lotstein probably has little to fear from this defense: no matter what Riccitelli might claim, canon law does not allow pastors to treat parish assets as their own property (1983 CIC 1256, 1281-1289) nor to enrich themselves at parochial expense. Canons 282 and 285-286 instruct clerics to shun ostentatious lifestyles and to avoid engaging in personal business transactions without the permission of the ordinary. And Canon 1344, n. 2, takes for granted that some canonical crimes are also civil crimes for which offenders might be prosecuted and punished by the state. In short, my guess is that a canonical expert would have an easy time refuting Riccitelli's apparent claim that canon law let him spend parish money any way he wanted, leaving a state prosecution of Riccitelli to stand or fall on the merits of the case. Plus, the possibility of examining canonical issues in other state cases where it might be relevant, nay possibly vital, would be neatly served.
The respect that canon law accords civil law (e.g., 1983 CIC 22) should be reciprocated. I am aware of the pitfalls awaiting those who venture the path between Church and State, but history has shown the passage to be possible. When properly followed, it can be of great benefit to the subjects of both legal systems.
*Some media sources describe Riccitelli as a "former pastor" others as a "former priest". Riccitelli's canonical status is not relevant to this topic of this post.