Something that is sacred is deemed to be dedicated to, or set apart for, the worship of a deity. A word describing something which is deemed sacred would be 'holy'.
One of the main reasons why the concept of sacredness is a threat to rationalism is the tendency toward dogma. That is, a belief which is sacred must not be questioned. The Bible is taken to be the infallible word of God and to doubt it or question it is to commit blasphemy or heresy. In rationalism, beliefs and claims should be questioned and examined. There is no assertion or claim which is above reproach or unable to be questioned.