Alma knows that Corianton had committed a serious sin: “I would to God that ye had not been guilty of so great a crime.” Alma gained that knowledge from personal experience, for in his rebellious time he had committed the same sin of leading the children of God astray. Alma also understood that repentance is possible from even that sin; he turns to working with Corianton in order to make repentance possible for Corianton.
The idea that one would “cross yourself in all these things,” is an awkward phrase of imprecise connotation. However, the clear meaning, in the context of verse 9, is to stop doing something that is not right. If it held the meaning that Catholics use to ascribe making the sign of the cross, it would have been to symbolically accept Christ’s atoning sacrifice.
It is unlikely that it meant making the sign of the cross, because the cross never became a religious symbol for the Nephites. Nephi saw that Christ would be crucified, but the transformation of the cross from instrument of death to symbol of resurrection is a New Testament process, and not one we see in the Book of Mormon. The Book of Mormon peoples never experienced the death of their God, only his impressive and glorious descent and appearance around the temple in the land of Bountiful (1 Nephi 11:1). The use of this “cross yourself” is most likely due to the more modern translation.