To begin, Meredith does not come across as an overly spiritual person. As her mother was a master of medicine, she too has come to invest more belief in science rather than religion. She has trouble dealing with emotions and showing love, which is perhaps why she is not spiritual.She has several experiences with death and patient's beliefs and she shows a sense of understanding and compassion toward them even if she does not necessarily share their points of view.
Izzie is a character who always has a "glass half full" mentality. Though she is an excellent doctor, she often lets emotions and beliefs get in the way of science. After she falls in love with her patient, Izzie believes he will not die on her because they are meant to be together. She utilizes both spirituality and science to try to save him while making some unethical decisions along the way. After his death, she has a hard time believing in science and spirituality.
Dr. Cristina Yang is a believer of science rather than religion. She is constantly striving for success in the operating room and has no time for spirituality or beliefs. She finds herself in a relationship with a resident, Dr. Burke. They share completely different religious views...he is spiritual, she is not. Cristina is a skeptic and will always put medicine over beliefs.