The blending of pagan elements in Christian art and doctrine helped convert pagans into new followers. It’s an effective enough idea. Bring in something familiar and the church seems less foreign and out of reach. The stories are more believable, the practices more easily endured. A dramatic change would be a scary change because most people are pretty well set in their beliefs. Easing people into Christianity by adopting certain elements of theirs helps makes the transition easy and doable.
Still, there are some potential backlashes. Christianity does not support paganism, and by absorbing such elements, they may seem somewhat hypocritical. It also makes the religion somewhat less believable because they borrowed ideas. Basically, it’s confusing to those who don’t know the history behind it or the reasons why pagan ideas are mixed with Christian ideas in certain pieces. Some new converts may not have truly believed the Christian faith the way it was intended because their art and beliefs still intermingled with it.
Whether or not the blending of pagan ideas in Christian art and doctrine was a good idea is debatable. I think it’s important to take into account that it resulted in some absolutely beautiful art (such as the Book of Kells). So despite the fact that it was a slightly underhand way of gaining followers, I appreciate the outcome in art.