The problem is, that's not how jury selection works.
In 1968, the United States Supreme Court decided the appeal of a man who had been given the death sentence by an Illinois court (Witherspoon v. Illinois). Potential jurors had been excluded on the basis of their general scruples against capital punishment. The Supreme Court said that deprived Witherspoon of a fair trial. For a successful challenge, prosecutors had to establish that a juror could never consider the death penalty in any case, or at any rate not in the case at hand.
By the same token, the defense must "reverse-Witherspoon" potential jurors to get rid of jurors who would certainly vote for the death penalty in the case. It is a similarly tough standard as the one for prosecutors. (One of the arts of capital defense is "rehabilitating" anti-capital-punishment potential jurors -- the ones the defense wants -- by getting them to agree that they would at least consider the death penalty.)
The practical result is that defendants will face a "Witherspooned" jury made of up people who have already sworn they will consider the death penalty.
There is absolutely no reason Dzhokhar Tsarnaev's jury shouldn't have Catholics on it. Without a doubt, many Catholics could consider a sentence of death. Catholic potential jurors will undergo voir dire like everyone else, and each one will be handled individually on the basis of his or her answers.