"Hypothetical situation: Let?s assume, for the sake of the argument, that the Constitution had a line in it that said 'Abortion shall be available to all.' It doesn?t ? this is a hypothetical question, a 'gedanken experiment.' That?s German for 'thought experiment.'"
"If Roberts were a Supreme Court justice and he were asked to rule on the constitutionality of a law that totally banned all abortions (remember, folks – 'gedanken'). The law is clearly unconstitutional under our hypothetical gedanken experiment. "
"As a Roman Catholic, would he be permitted to rule that the Constitution says the law is unconstitutional? Or would he, because of his faith, be required to rule that the law is constitutional?"
Yes, he would be permitted to do so. However, if doing so would bother his conscience, he has two other options. He could either recuse himself before hearing the case or abstain from voting after hearing it.
"If he ruled that the law was constitutional, would he be banned from communion or even excommunicated?"
No, he would not be banned or excommunicated on the basis of his vote alone. Interpretation of constitutional law – is it legal – is not the same as making or defending law – should it be legal. Defending the legality of abortion, on the other hand, might land him in hot water.
"If he must rule a certain way, no matter what the evidence, is he capable of doing the job?"
This is a moot question given the fact that he's free to support or deny the constitutionality of any law so long as he does so honestly and without bias.
"Should it matter if a judge is incapable of ruling according to what the law actually says?"
As opposed to what? Are we talking denotation or connotation? letter or spirit? expression or intention?"What the law actually says" is often up to interpretation. Legalese is notoriously riddled with loopholes and wiggle room (The much-abused "pastoral reasons" in canon law is a prime example.). As I mentioned the other day, a lot of people seem to treat the Constitution like they treat Holy Scripture. That is, it's infallible in every single word and the letter of the law always trumps the spirit. Then again, others seem to treat it, much as they do Scripture, as warm and fuzzy platitudes written in invisible ink and forever open to revision by anyone with a pet cause.
While we're talking about Roberts, I'd like to point people to a post by Nathan at Quo Vadis. It's about how little we really know about Roberts and how he might end up being a disappointment to conservatives like Souter is. I don't always agree with Nathan (especially on homosexuality), but I always find him interesting. Be sure to check out his post.
Also, by popular demand, Jimmy Akin has a Roberts post for people to comment on.