“Some readers of the Book of Mormon have wondered about Moroni’s statement that the elder or priest administering to the sacrament “did kneel down with the church.” (Moroni 4:2.) Some have assumed the meaning that everyone in the congregation knelt down. Although this interpretation is possible, at least one other possibility exists. The statement may mean that the elder or priest did kneel down “in the presence of the members of the church.” (See also D&C 20:76.)” (Daniel H. Ludlow, A Companion to Your Study of the Book of Mormon [Salt Lake City: Deseret Book Co., 1976], 332.)
Regardless of how we interpret the body of those who knelt, it is certain that someone was kneeling, and that it was, at a minimum, the administrator. Along with the liturgical language, Moroni is describing liturgical actions. Similar to the use of the hands to symbolically transmit spiritual authority (for the Gift of the Holy Ghost or the ordination to the priesthood) the kneeling is a form that represents the proper way to accomplish the rite of the sacrament.
Why kneeling? Ultimately, we do not know. However, we do know that in the Book of Mormon the physical expression of falling to the ground was an indication of a reaction to a spiritual experience. The people of Benjamin fall to the ground (Mosiah 4:1), Alma the Younger falls to the earth (Mosiah 27:18), and the king of the Lamanites falls to the earth (Alma 22:18). While those occasions indicated an overwhelming spirituality by prostration, it would seem that the connection of the body and the earth held a connotation of sacred movement or relationship. The kneeling would invoke those same cultural sentiments, as well as humility. This pre-Messianic-visit form may have been transformed into the kneeling posture that was used several times during that occasion. Kneeling is specifically requested of the Nephites during the Savior’s appearance in Bountiful:
3 Nephi 17:13-15
13 And it came to pass that when they had all been brought, and Jesus stood in the midst, he commanded the multitude that they should kneel down upon the ground.
14 And it came to pass that when they had knelt upon the ground, Jesus groaned within himself, and said: Father, I am troubled because of the wickedness of the people of the house of Israel.
15 And when he had said these words, he himself also knelt upon the earth; and behold he prayed unto the Father, and the things which he prayed cannot be written, and the multitude did bear record who heard him.
3 Nephi 19:6
6 And the twelve did teach the multitude; and behold, they did cause that the multitude should kneel down upon the face of the earth, and should pray unto the Father in the name of Jesus.
3 Nephi 19:16
16 And it came to pass that he spake unto the multitude, and commanded them that they should kneel down again upon the earth, and also that his disciples should kneel down upon the earth.
In each of these occasion there is a prayer of a multitude. Since in each of these occasions there is a mass kneeling, it would not be surprising if the congregation did kneel as well as the officiator in the ordinance of the sacrament.