In Mosiah 15:9, we also say that the atonement would straddle the demands of “justice” and “mercy.” Those are two terms that the Book of Mormon assumes that we understand, as they are not defined. Justice applies the penalties of the law, according to the law. Mercy allows for circumstances that might allow the penalties of the law to be voided. Because both justice and mercy relate to the law, they are difficult to reconcile. It is difficult to have justice and mercy simultaneously.
Amulek teaches that the infinite atonement reconciles the two by providing a vicarious acceptance of the penalty of the law, while allowing humankind access to mercy, which is allowed, because of the vicarious satisfaction of that justice.
Thus, humankind has access to both the benefits of the law (justice), and the wisdom of mercy. Humankind can repent, and specifically, the people to whom Amulek is preaching can repent; they can “begin to exercise … faith unto repentance.”
Amulek notes that, because of the principle of repentance, they are able to call upon their God to apply the atonement. Having noted that they might begin repenting, he will next explain how they might do that.