The phrase "every good thing" shows up six times in the Book of Mormon, in ten consecutive verses of Moroni 7 (the color code scheme I am using will make sense a little later in this post):
19 Wherefore, I beseech of you, brethren, that ye should search diligently in the light of Christ that ye may know good from evil; and if ye will lay hold upon every good thing, and condemn it not, ye certainly will be a child of Christ.
20 And now, my brethren, how is it possible that ye can lay hold upon every good thing?
21 And now I come to that faith, of which I said I would speak; and I will tell you the way whereby ye may lay hold on every good thing.
22 For behold, God knowing all things, being from everlasting to everlasting, behold, he sent angels to minister unto the children of men, to make manifest concerning the coming of Christ; and in Christ there should come every good thing.
23 And God also declared unto prophets, by his own mouth, that Christ should come.
24 And behold, there were divers ways that he did manifest things unto the children of men, which were good; and all things which are good cometh of Christ; otherwise men were fallen, and there could no good thing come unto them.
25 Wherefore, by the ministering of angels, and by every word which proceeded forth out of the mouth of God, men began to exercise faith in Christ; and thus by faith, they did lay hold upon every good thing; and thus it was until the coming of Christ.
26 And after that he came men also were saved by faith in his name; and by faith, they become the sons of God. And as surely as Christ liveth he spake these words unto our fathers, saying: Whatsoever thing ye shall ask the Father in my name, which is good, in faith believing that ye shall receive, behold, it shall be done unto you.
27 Wherefore, my beloved brethren, have miracles ceased because Christ hath ascended into heaven, and hath sat down on the right hand of God, to claim of the Father his rights of mercy which he hath upon the children of men?
28 For he hath answered the ends of the law, and he claimeth all those who have faith in him; and they who have faith in him will cleave unto every good thing; wherefore he advocateth the cause of the children of men; and he dwelleth eternally in the heavens.
It's really hard to summarize these words, since so many themes are swirling around each other. Here's my attempt:
Those who exercise faith and cleave to every good thing (all of the covenants) given to them by ministering angels will become a child of Christ and lay hold on every good thing (all that the Father has).
This is the oath and covenant of the priesthood, using different words!
33 For whoso is faithful unto the obtaining these two priesthoods of which I have spoken, and the magnifying their calling, are sanctified by the Spirit unto the renewing of their bodies.
34 They become the sons of Moses and of Aaron and the seed of Abraham, and the church and kingdom, and the elect of God.
35 And also all they who receive this priesthood receive me, saith the Lord;
36 For he that receiveth my servants receiveth me;
37 And he that receiveth me receiveth my Father;
38 And he that receiveth my Father receiveth my Father’s kingdom; therefore all that my Father hath shall be given unto him.
39 And this is according to the oath and covenant which belongeth to the priesthood.
40 Therefore, all those who receive the priesthood, receive this oath and covenant of my Father, which he cannot break, neither can it be moved.
41 But whoso breaketh this covenant after he hath received it, and altogether turneth therefrom, shall not have forgiveness of sins in this world nor in the world to come.
42 And wo unto all those who come not unto this priesthood which ye have received, which I now confirm upon you who are present this day, by mine own voice out of the heavens; and even I have given the heavenly hosts and mine angels charge concerning you.
43 And I now give unto you a commandment to beware concerning yourselves, to give diligent heed to the words of eternal life.
44 For you shall live by every word that proceedeth forth from the mouth of God. (D&C 84)
As if to underscore this point even further and make it even more clear that "every good thing" is a reference to making and keeping our covenants, compare verse 22:
22 For behold, God knowing all things, being from everlasting to everlasting, behold, he sent angels to minister unto the children of men, to make manifest concerning the coming of Christ; and in Christ there should come every good thing. (Moroni 7)
...with verses 31-32:
31 And the office of their ministry (i.e., the ministry of angels) is to call men unto repentance, and to fulfil and to do the work of the covenants of the Father, which he hath made unto the children of men, to prepare the way among the children of men, by declaring the word of Christ unto the chosen vessels of the Lord, that they may bear testimony of him.
32 And by so doing, the Lord God prepareth the way that the residue of men may have faith in Christ, that the Holy Ghost may have place in their hearts, according to the power thereof; and after this manner bringeth to pass the Father, the covenants which he hath made unto the children of men. (Moroni 7)
I posted quite a bit about "the Office of the Ministry of Angels" recently. (Overview of seven-part series here.)
This is a very exciting discovery for me. Once again, we see that an expression with a pleasant ring to it ("every good thing") is actually a symbol of our covenants, mercifully hidden beneath the surface layer of meaning.
Moroni takes one more step to make this connection abundantly clear in the final verses of the Book of Mormon:
30 And again I would exhort you that ye would come unto Christ, and lay hold upon every good gift, and touch not the evil gift, nor the unclean thing.
31 And awake, and arise from the dust, O Jerusalem; yea, and put on thy beautiful garments, O daughter of Zion; and strengthen thy stakes and enlarge thy borders forever, that thou mayest no more be confounded, that the covenants of the Eternal Father which he hath made unto thee, O house of Israel, may be fulfilled.
32 Yea, come unto Christ, and be perfected in him, and deny yourselves of all ungodliness; and if ye shall deny yourselves of all ungodliness, and love God with all your might, mind and strength, then is his grace sufficient for you, that by his grace ye may be perfect in Christ; and if by the grace of God ye are perfect in Christ, ye can in nowise deny the power of God.
33 And again, if ye by the grace of God are perfect in Christ, and deny not his power, then are ye sanctified in Christ by the grace of God, through the shedding of the blood of Christ, which is in the covenant of the Father unto the remission of your sins, that ye become holy, without spot. (Moroni 10)
Below is my summary of Moroni 7:19-28 again. I hope I've convinced you this language is pointing us to the covenant.
Those who exercise faith and cleave to every good thing (all of the covenants) given to them by ministering angels will become a child of Christ and lay hold on every good thing (all that the Father has).
Also relevant are the words given to Joseph Smith in June 1829, probably around the same time he was completing the Book of Mormon translation:
6 Seek to bring forth and establish my Zion. Keep my commandments in all things.
7 And, if you keep my commandments and endure to the end you shall have eternal life, which gift is the greatest of all the gifts of God. (D&C 14)
I genuinely wonder what these words (D&C 14, Moroni 7, and Moroni 10) meant to Joseph Smith in 1829. How much did he understand about what was to come? Mormon and Moroni seem to understand very clearly what the temple covenant really means, but this does not seem to have been made explicit to Joseph Smith until the revelation of D&C 84 in 1832, and the earliest temple of the restoration was still 4 years away at that time (and the endowment was about ten years away).
This is all very interesting. If Joseph Smith was the author of the Book of Mormon, he was working an incredibly long con!