The road to apostasy is the road of rationalizing those things that are offensive to the Holy Ghost. This is not only true of individuals, it is also true collectively.
And why is it that? Because when a member, or a family, or a quorum, or a ward, finds themselves misbehaving in little, but potent, ways frequently enough then they find little connection to the powers of heaven. At that point not only have they lost their guide, but they also experience no difference, no loss, when they increase their indulgences. They soon rationalize that the greater sins are no worse than their former ones. This path is a dark spiral downward that has no end but tragedy. When they reject the one lesson referred to in D&C 121, believing that they are connected to the powers of heaven on new and more lenient principles than God’s own principles of righteousness, they are headed on a path for destruction. That is why Joseph F Smith spoke of the three threats to the church from within. They are those things that cut men off from the Holy Ghost and put him under the influence of the adversary, and replace the one lesson with the adversary’s lesson: that we can find ourselves connected to the powers of heaven on any terms other than God’s own principles of righteousness.