Which christian doctrine does the catechism say meets more opposition than any other?

The resurrection of the body. 

The Christian doctrine of a physical resurrection, based on the reality of Jesus' resurrection, not only meets with skepticism from non-Christians, but even within Christianity itself. A survey conducted in 2006 by the Scripps Survey Research Center at Ohio University found that only 36% of Americans believed that after death, their body will be resurrected someday. A 2017 survey conducted in Britain by the BBC found that one-quarter of Christians there said they did not believe Christ actually rose from the dead, and 31% of Christians in that survey said they did not believe in life after death. Among those who do believe in life after death, often they are thinking in terms of an afterlife for the soul, not the body.

after the civil war, and with the termination of slavery, the south was faced with a big problem; what was supposed to happen and how they were supposed to deal with the freedom and the rights that black people and freed slaves had gained as a result of the north defeating the south and forcing the end of slavery. first, most, if not all, of the workforce of the south came from slaves and they did not have an option to replace the loss of said workforce. second, particularly in the south, people were convinced that black people were lesser, less intelligent, less capable and less smart than whites. this posed a huge problem for them. also, southerners were faced with another issue; freed slaves now had a say in how their lives would be lived and the right to politically choose the way they would be governed. people in the south did not want to accept this, none of this and this opposition and the conflicts that arose from it, not to mention the afereffects of the war, forced the congress of the time to enact and enforce the reconstruction plans that had been proposed by president andrew johnson.