Verses 9 and 10 are Mormon’s personal testimony about the importance of the record he is writing. He has included the information from the visit of the Savior that concentrates on the written record and its role in the future history. As the creator of this particular record, Mormon cannot help but add his perspective on the work in which he is involved.
[and when they shall have received this]: “They” are the Gentiles, and “this” is the Book of Mormon. In this context, the modern Latter-day Saints are the Gentiles.
[which is expedient that they should have first]: The Book of Mormon comes first to the Gentiles, and from them to the House of Israel, including the descendants of the people of the book.
[to try their faith, and if it shall so be that they shall believe these things then shall the greater things be made manifest unto them.]: Whenever we see the phrase “try faith” is it best to see it in the light of the Psalm:
Psalms 66:10
10 For thou, O God, hast proved us: thou hast tried us, as silver is tried
In a modern English connotation of the phrase “try faith” it might appear to be an intentional attempt to damage faith by providing a difficult test. The purpose of the Lord is never to damage our faith, but to refine it. When our faith is “tried” as is silver, it is purified through the heat of the refiner’s fire. There may be difficulties, but they exist to polish and purify our faith, not to see if we might lose it.
[if it so be that they will not believe these things, then shall the greater things be withheld from them]: The opportunity of the Gentiles with respect to the Book of Mormon is the same promise the Lord always makes to his children. For those who may be faithful in a few things, greater things may be added. This is the entire focus of the parable of the talents:
Matthew 25:14-28
14 ¶ For the kingdom of heaven is as a man travelling into a far country, who called his own servants, and delivered unto them his goods.
15 And unto one he gave five talents, to another two, and to another one; to every man according to his several ability; and straightway took his journey.
16 Then he that had received the five talents went and traded with the same, and made them other five talents.
17 And likewise he that had received two, he also gained other two.
18 But he that had received one went and digged in the earth, and hid his lord’s money.
19 After a long time the lord of those servants cometh, and reckoneth with them.
20 And so he that had received five talents came and brought other five talents, saying, Lord, thou deliveredst unto me five talents: behold, I have gained beside them five talents more.
21 His lord said unto him, Well done, thou good and faithful servant: thou hast been faithful over a few things, I will make thee ruler over many things: enter thou into the joy of thy lord.
22 He also that had received two talents came and said, Lord, thou deliveredst unto me two talents: behold, I have gained two other talents beside them.
23 His lord said unto him, Well done, good and faithful servant; thou hast been faithful over a few things, I will make thee ruler over many things: enter thou into the joy of thy lord.
24 Then he which had received the one talent came and said, Lord, I knew thee that thou art an hard man, reaping where thou hast not sown, and gathering where thou hast not strawed:
25 And I was afraid, and went and hid thy talent in the earth: lo, there thou hast that is thine.
26 His lord answered and said unto him, Thou wicked and slothful servant, thou knewest that I reap where I sowed not, and gather where I have not strawed:
27 Thou oughtest therefore to have put my money to the exchangers, and then at my coming I should have received mine own with usury.
28 Take therefore the talent from him, and give it unto him which hath ten talents.
In this parable, those who are successful husbands of what they have been given receive an increase. The one who was not had that which had been given taken away and given to another. The promise of potential greater things is a promise given implicitly to Joseph Smith as the translator of Mormon’s record. The experience of the restoration which followed the translation of the Book of Mormon suggests that there were indeed many “greater things” that were given after the coming forth of the Book of Mormon.