Jacob clearly declares that what is required is an infinite atonement, but what does he mean? The Nephites have been taught the law of Moses. They would have seen the sacrifices that were made, and particularly those for atonement. Israel regularly atoned for communal sin by having a sacrifice made. Thus, they understood the concept of a specific sacrifice to be made for the collective sins of the people. That sacrifice, however, was specific to the communal sins of the time, and future communal sins required a future sacrifice.
Humankind was in a situation where all were subject to sin, in the past, in the present, and in the future. For that type of collective sin, no animal sacrifice would suffice, no matter how often it was made. Therefore, it had to be an infinite atonement, an act of sacrifice once that would cover all humankind through all mortal times.
The first judgment upon humankind, through Adam and Eve, was death. Without a resurrection, death would have been permanent.