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

The answer to this is Resurrection of the Body. Bible says that when Jesus returns to earth, he will physically raise all those who have died, giving back the bodies they have lost at death. Resurrection bodies will not die and for the righteous, they will be transformed into a glorified state.

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.