I think their religion is very relevant to both sides. For the Prosecution, it shows the lengths Jodi went to to be "like" Travis (her conversion) and how she took his rejection of her for not being good enough/marriage material. It goes to motive. For the defense, they are trying to use it to show how "deep" and committed she was to the church, and how badly Travis treated her for indulging in the same behaviors he did (each with the other) and how he had the charisma and power to mentally enslave her (their take on it, not mine). All JMO.