Daniel H. Ludlow comments:
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” (Moro. 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.)
Ludlow’s interpretation seems to rest on modern practice. Both the Book of Mormon text and Doctrine and Covenants 20:76 specify that the priest “shall kneel with the church and call upon the Father.”
Along with the liturgical language, Moroni is describing ritual actions. Like the laying on of hands to transmit spiritual authority (e.g., the gift of the Holy Ghost or priesthood ordination), kneeling is a physical posture that represents the proper position in which to perform the rite of the sacrament.
Why kneeling? Ultimately, we do not know. However, in the Book of Mormon, falling to the ground was a frequent reaction to a spiritual experience. The people of Benjamin fall to the ground (Mosiah 4:1), Alma2 falls to the earth (Mosiah 27:18), and the Lamanite king falls to the earth (Alma 22:18). While such prostration may indicate being overwhelmed by the Spirit, it seems logical that prostration (or physically lowering oneself to touch the earth) connoted humility and recognition of the sacred. Several times, the Nephites knelt during the Savior’s visit:
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.
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.
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 Ne. 17:13–15)
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 Ne. 19:6)
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. (3 Ne. 19:16)
On all three occasions, a group prays and does so kneeling. Thus, it seems reasonable that, at the supremely sacred moment of the sacrament, the congregation as well as the officiator would also kneel.