The people at this temple were invited to come forward, one at a time, and see for themselves, and feel Christ’s wounds with their own hands. In doing so, they were able to internalize the reality of the crucifixion and death of Jesus as well as the reality of His resurrection. After participating in this amazing experience, “they did cry out with one accord” (3:16). Most likely, somebody initiated and led the chorus. However, they all knew this hosanna shout—it was part of their ancient temple liturgy. They knew this from Psalms 118:25.
There was no better expression for these people to use in welcoming Jehovah, the Lord they had worshipped, into their midst—“Hosanna! Blessed be the name of the Most High God!” The ancient interpretation of the word “hosanna” means “save us now.” “Hosanna” was used to respectfully address the God who saves, saved, will save, or is saving now. They had heard the Savior explain that He had completed the mission He was foreordained to accomplish. He had suffered greatly—“drunk out of that bitter cup”— to take upon Himself the sins of the world to save all mankind (11:11). They had felt and seen the scars that represented the depth of His divine suffering and understood that this had been done for them personally. The hosanna shout was the best expression of their deep love and gratitude to the Lord. The hosanna psalm continues, “God is the Lord, which hath shewed us light; … Thou art my God, and I will praise thee; … for his mercy endureth for ever” (Psalms 118:27–29).
The people then fell down at the feet of Jesus and worshipped Him. Anciently, people fell down in the presence of the high priest. There are many reasons why the people fell down before Jesus Christ. One likely explanation (evidenced by the fact that this took place in a ritual setting at the temple) is that the people were acknowledging Jesus as the Great High Priest. They were acclaiming Christ to be the true Eternal High Priest.
Book of Mormon Central, “Why did the People Fall Down at the Feet of Jesus? (3 Nephi 11:17),” KnoWhy 202 (October 5, 2018).