This is one of the eccentricities of criminal law.
Had the jury used a Bible in determining guilt during the trial phase, it would not have been lawful, according to a lot of precedent, because Biblical text is not statutory law, but can be confused for statutory law. However, at the same time, jurors have great flexibility as to how they can personally decide guilt or innocence, even with the use of a coin flip.
That is, Bible no, coin flip yes, for guilt or innocence.
But this changes entirely with the sentencing phase. The jury is not acting based on “reasonable doubt”, but on their subjective opinion as to what is an appropriate punishment. And the Bible can be used in this case, along with anything else juries wish to use to discuss the severity of punishment. That can decide death instead of life based on the appearance of the guilty defendant, if they want to.
You beat me to it. That is exactly what I was thinking. We're asking people to use their morality and sense of justice in sentencing, so why shouldn't some people be able to use the reference materials for their source of morality and sense of justice?