I had noticed this verse before, but it struck me again this time what a unique clarification of the meaning of spiritual death Alma gives to the people of Ammonihah:
And therefore, he that will harden his heart, the same receiveth the lesser portion of the word; and he that will not harden his heart, to him is given the greater portion of the word, until it is given unto him to know the mysteries of God until he know them in full.
And they that will harden their hearts, to them is given the lesser portion of the word until they know nothing concerning his mysteries; and then they are taken captive by the devil, and led by his will down to destruction. Now this is what is meant by the chains of hell.
. . .
And now behold, I say unto you then cometh a death, even a second death, which is a spiritual death, then is a time that whosoever dieth in his sins, as to a temporal death, shall also die a spiritual death; yea, he shall die as to things pertaining unto righteousness.
-Alma 12:10-11, 16
Joseph Smith taught, "A man is saved no faster than he gets knowledge." I find it really interesting that the process Alma describes captivity as losing knowledge of God. It makes sense that if knowing Heavenly Father and Jesus Christ constitutes life eternal, that losing knowledge of them would constitute spiritual death, or hardening the heart such that there is no room for righteousness. More than the quantity of our studies, the condition of our hearts will determine the portion of eternal knowledge that we will have the capacity to receive, and that in turn will determine our eternal destiny.