Death Note incorporates two iconic Christian images into the first opening of the series. Namely the creation of Adam and the Pietà.
In the same way that in Michelangelo's painting the creation of Adam is said to depict god creating a new life, the first man, the Death Note version shows a God of Death handing Light an apple (often used as a stand in representing the Death Note) once again giving life to Kira.
The Pietà depicts a sculpture of Mary cradling Jesus' body after the crucifixion while Death Note's version shows Naomi cradling the body of her fiancé, Raye Penber, who had been killed by Kira. The image captures the tone of the series at this point. While heartbroken, Naomi handles the death of her fiancé very well with more grace than expected. It is also the first time that Light killed an innocent person, up to this point he had only killed criminals, similar to the innocent Jesus dying it marked a turning point in the series.
The manga covers also employ christian imagery from the image of the cross, the posture of characters, radiating light, stain glass windows reminiscent of church windows, images of the virgin Mary and many others are depicted regularly on the covers of each issue.
This shows us that the inclusion of religious element have been a conscious decision from the beginning and important enough to the series that it was depicted on every manga cover.