The word “firstlings” puts certain qualifications and restrictions and even determines the quality of faith that is used in offering the sacrifice. “Firstling” does not necessarily denote the oldest of the flock, but the firstborn of its particular mother. A “firstling” is a male, the “first to open the matrix” of its mother (Ex. 13:2; 34:19). Each mother in her lifetime could produce only one firstling, but a flock of sheep could have several firstlings born each year. In order to know which lambs were acceptable for sacrifice, the owner would have to know his flock. Some notice would have to be made of mothers and of young. Otherwise, how could anyone know which mothers had produced offspring for the first time? There is no way that a man, Adam or anyone else, could know which males were firstlings unless a record and some identification of mothers and offspring were kept. This requirement removes the element of chance and of haphazard obedience, or sometime obedience. Not only is one’s faith shown in the willingness to offer a sacrifice but also in the care required and the preparation needed beforehand in making the selection of the proper animal.
This particular passage of scripture illustrates the concept that the commandments of God require intelligent and deliberate attention of those who are seeking salvation. It gives a reason for Paul’s observation that “without faith it is impossible to please” God (Heb. 11:6). For without faith one would not have kept a record and marked (at least mentally) which animals were proper for sacrifice. (Robert J. Matthews, “The Doctrine of the Atonement,” in Studies in Scriptures, Volume Two: The Pearl of Great Price, ed. Robert L. Millet and Kent P. Jackson 118-19.) — The Pearl of Great Price Student Manual, pp. 15-16