Alma 36

As Alma teaches his son, Helaman concerning the process of repentance and the power of the atonement, he testifies -

"...I swear unto you, that inasmuch as ye shall keep the commandments of God ye shall prosper in the land." (Book of Mormon | Alma 36:1)

Many would interpret "prosper" to mean blessed materialistically. But Alma continues to explain a better understanding of prosper telling his son -

"I would that ye should do as I have done, in remembering the captivity of our fathers; for they were in bondage, and none could deliver them except it was the God of Abraham, and the God of Isaac, and the God of Jacob; and he surely did deliver them in their afflictions." (Book of Mormon | Alma 36:2)

The emphasis here is of those that did keep His commandments, the Lord "did deliver them in their afflictions". He explains that to be delivered from afflictions means -

"...for I do know that whosoever shall put their trust in God shall be supported in their trials, and their troubles, and their afflictions, and shall be lifted up at the last day." (Book of Mormon | Alma 36:3)

Prosper would then suggest deliverance. The Lord's perspective of deliverance is that we are supported in trials, troubles, and afflictions. It does not mean the we will not have trials, troubles and afflictions. And Alma testified -

"...I would not that ye think that I know of myself—not of the temporal but of the spiritual..." (Book of Mormon | Alma 36:4)

Meaning that through the atonement, Alma learned this of himself, for -

"...because of the word which he has imparted unto me, behold, many have been born of God, and have tasted as I have tasted, and have seen eye to eye as I have seen; therefore they do know of these things of which I have spoken, as I do know; and the knowledge which I have is of God.
27 And I have been supported under trials and troubles of every kind, yea, and in all manner of afflictions; yea, God has delivered me from prison, and from bonds, and from death; yea, and I do put my trust in him, and he will still deliver me." (Book of Mormon | Alma 36:26 - 27)

Maybe this is what the Lord meant when He used the words "prosper in the land".