1. For believers, Gospel may be perceived as a guideline that would help them to increase the overall quality of their life and the people around them. For non-believers, Gospel often perceived as a distortion of Truth for Fanatical Heretics that are afraid to face reality.
2. Probably about some part of Gospel that show how Christians should treat their slaves. This part of Gospel make many people believe that the Church is in support of slavery (no direct condemnation of it), and this could be made as a moral reason to reject Gospel message.
3. The main emotional reason would be because it is hard for most humans to believe something that cannot directly be proven in front of them. This create some sense of doubt regarding the validity of the Gospel and cause them to eventually decide to reject it instead.
4. Because some occurrence in Christian Gospel are considered to be impossible if observed using scientific standards (at least for now) Without actual concrete formulation or evidence, it is very hard for intellectual to admits the occurences that happened in Gospel.
5. The only things that Christian could do is relieve the Gospel themselves. Faith is something that must be achieved with personal efforts and couldn't be forced onto others. By reliving the Gospel and improving their quality of life, they could live as an example that might influence others people to follow
