A Christian Scientist accused of manslaughter for not seeking medical help for his son's fatal ailment testified Tuesday that he had believed prayer was helping the boy. Special Prosecutor John Kiernan cross-examined David Twitchell in an effort to uncover inconsistencies in prior testimony about the severity of the five-day illness of Twitchell's 2{-year-old son Robyn, who died four years ago of a bowel obstruction. Both Twitchell and his wife, Ginger, both 34, are accused of manslaughter. The Boston-based Christian Science Church teaches that disease, like sin, can be healed by spiritual means alone. Twitchell testified he was concerned over his son's health but believed that prayer was working for Robyn, even during the last few hours of his life. Kiernan again questioned Twitchell on whether he made a mistake in not taking his son to doctors when he began vomiting and gesturing at his stomach. ``If they could hawhy you didn't call a doctor for Robyn, because you wanted to maintain your spiritual purity?'' Kiernan asked Tuesday. ``Absolutely not,'' Twitchell replied. Under questioning by defense attorney Rikki Klieman, Twitchell testified about his knowledge of legal obligations for a Christian Scientist. For example, he said, Christian Scientists were not exempt from required prenatal blood tests, such as one given to his wife, but that school children did not have to be immunized. Prosecutors have emphasized they are not putting the Christian Science faith on trial, but that they are seeking to protect children placed at risk by parents who rely solely on spiritual healing.