The Savior is referring to the commandment received of the Father to warn the Gentiles of the last days. This theme was taught in 3 Nephi 16:8-20, but he could not continue because they were weak in understanding (3 Nephi 17:2). The Savior had more to say and he did not fulfill this commandment until all of his sermon (comprising 3 Nephi 20-22) was given. The amount of time he gave to this topic and the Father's specific commands regarding it underscores its importance.