What's happening
This is the revelation of the founding day itself — given on April 6, 1830, in the same room and the same hour the Church was organized.
The scene is the Peter Whitmer Sr. farmhouse in Fayette, New York. A small group had gathered, six men formally constituting the new Church under the laws of New York (a number of others were present besides). They had the Book of Mormon, freshly printed; they had the Articles and Covenants that Section 20 records; they had the priesthood restored the year before. What they did that day was simple and momentous: they organized, partook of the sacrament, baptized and confirmed, and ordained one another. And in the middle of it Joseph received this revelation, which David Whitmer later remembered as the word that Joseph "should be the leader; that he should be ordained by Oliver Cowdery as 'Prophet Seer and Revelator,' and that the church should receive his words as if from God's own mouth."
So Section 21 is the charter of the prophetic office. Section 20 organized the institution; Section 21 tells the institution how to relate to the man through whom it receives revelation. It opens by piling titles on Joseph — a seer, a translator, a prophet, an apostle of Jesus Christ, an elder of the church (v. 1) — and it closes by having Oliver and Joseph ordain each other into their offices before the gathered body. It is, in the most literal sense, the Church learning on day one how it will be led.
(A small note on place, because the record is honest about it: the earliest manuscript gives Fayette as the location, but the first printing in 1833 changed it to Manchester — apparently a slip by W. W. Phelps as he prepared the Book of Commandments. The Joseph Smith Papers restore the original Fayette reading. It is a useful reminder that even the founding documents passed through human hands and human errors, exactly as Section 1 said the Lord's word would — given after the manner of their language.)
What gives the section its warmth, though, is not the titles. It is verses 7 through 9, where the Lord turns from office to the man's interior life and speaks of Joseph's weeping for Zion — and promises he shall mourn for her no longer. The founding-day revelation pauses, in the middle of establishing an institution, to comfort a young man who has been carrying it in tears.
A seer, a translator, a prophet (verses 1–3)
Behold, there shall be a record kept among you; and in it thou shalt be called a seer, a translator, a prophet, an apostle of Jesus Christ, an elder of the church through the will of God the Father, and the grace of your Lord Jesus Christ. — D&C 21:1
The very first words establish two things at once, and it is easy to rush past the first. There shall be a record kept among you. Before the Lord names Joseph's office, He commands a history. The Church is, from its first sentence, a record-keeping people — and the command will be formalized soon after when John Whitmer is appointed to keep the church record and history (Section 47). This is the same instinct that runs through the whole Book of Mormon, the conviction that what God does in a generation must be written so the next generation can be held by it. The Restoration began with a buried record; it organizes itself by commanding a new one.
Then the titles. Seer, translator, prophet, apostle, elder. The order matters. Seer comes first, and it is the highest of them. The Book of Mormon had already defined the word: when Ammon explains the seership of Mosiah, he says a seer is greater than a prophet... a seer is a revelator and a prophet also (Mosiah 8:15–16), one who can know things past and things to come and bring hidden things to light. The man who translated that definition is now given the office it describes. Translator records what he has just done with the plates; prophet what he will keep doing; apostle binds him to the New Testament office of a sent witness; elder sets him inside the church's own order, one officer among others. The titles ascend from the cosmic gift down to the local office — and all of it, the verse insists, through the will of God the Father, and the grace of your Lord Jesus Christ. The man is exalted and immediately told that the exaltation is grace.
His word ye shall receive (verses 4–6)
Wherefore, meaning the church, thou shalt give heed unto all his words and commandments which he shall give unto you as he receiveth them, walking in all holiness before me; For his word ye shall receive, as if from mine own mouth, in all patience and faith. — D&C 21:4–5
Here is the verse that has done the most work in Latter-day Saint life, and it is worth reading exactly, because it is more careful than it is sometimes quoted. The Church is to receive the prophet's word as if from mine own mouth — but notice the two qualifiers built right into the sentence. The words to be received that way are the ones which he shall give unto you as he receiveth them — that is, the revelations he gets from God, not his every private opinion. And the receiving is to be in all patience and faith, which is the posture of a disciple, not the reflex of a servant. This is not a blank check; it is a covenant of trust around the channel of revelation. The Lord is telling a brand-new church that He will lead it through a living voice, and asking it to honor that voice even when it must be received slowly, in patience.
And the promise attached is the one the Restoration keeps reaching for:
For by doing these things the gates of hell shall not prevail against you... — D&C 21:6
That sentence — Christ's promise to Peter (Matthew 16:18) — has now been handed, across Sections 17, 18, and 21, to the witnesses, to the apostles, and now to the whole Church gathered around its prophet. The thing that keeps the gates of hell from prevailing is not the prophet's brilliance; it is the Church's willingness to give heed to revelation and walk in all holiness. Heeding the living word is the wall.
His weeping for Zion (verses 7–9)
Then the revelation does something tender and unexpected. Having established the office, it turns to the man's heart:
Yea, his weeping for Zion I have seen, and I will cause that he shall mourn for her no longer; for his days of rejoicing are come unto the remission of his sins... — D&C 21:8
On the day he is publicly named prophet and seer before the gathered Church, the most personal thing the Lord says to Joseph is that He has seen his weeping. The young man carrying the whole improbable weight of this — the ridicule, the poverty, the lost pages, the long translation, the wondering whether any of it would hold — has been weeping for Zion in private, and the Lord notices the tears before He notices the titles. His days of rejoicing are come unto the remission of his sins. The founding-day revelation, in the middle of organizing an institution, stops to comfort one grieving person by name. It tells you what kind of God is at the head of this church: one who builds a kingdom and, in the same breath, dries one man's eyes.
The mutual ordination (verses 10–12)
Wherefore it behooveth me that he should be ordained by you, Oliver Cowdery mine apostle... — D&C 21:10
The section ends with a beautiful piece of order. Joseph is to be ordained by Oliver, and Oliver is an elder under his hand — the two ordain each other, each receiving his office from the other's hands. Neither man self-appoints. Even the prophet does not lay hands on himself; he is ordained by his fellow, and then ordains his fellow in turn. Authority in this church, from the first hour, passes from hand to hand by ordinance — never seized, always conferred. Oliver becomes the first preacher of this church (v. 12), and the gospel's reach is named in the order it will actually travel: to the Church, then before the world, then to the Gentiles; yea, and... to the Jews also.
How the section is built
The structure moves from office to man to order, and the middle is the surprise. It opens with the titles and the record (vv. 1–3) — the public, institutional naming of the prophet. It moves to the covenant of heed (vv. 4–6) — how the Church will relate to revelation, with the gates-of-hell promise as the seal. Then, at the center, it drops into the personal (vv. 7–9) — the Lord's notice of Joseph's diligence, prayers, and weeping. And it closes with the ordinance (vv. 10–12) — the mutual ordination that puts the whole thing under the law of conferred authority. A revelation that could have been all office is built around a moment of comfort, and a revelation that exalts one man ends by making him receive his office from another's hands. Exaltation and humility, in the same twelve verses.
How this sits in the documentary record
Section 21 is anchored to a precise, well-attested day, and the participants left accounts of what was said and done.
Where this lands
If Section 20 gave the Church its skeleton, Section 21 gave it its heartbeat: a living voice. The deepest claim of the section is that this church would not run on a finished book alone but on continuing revelation through a prophet — and that the people's part is to give heed... in all patience and faith. That is a demanding trust, and the section is careful not to make it blind: the word to be received as God's own is the revealed word, received patiently, by a people walking in all holiness.
But hold onto verse 8. The center of the founding-day revelation is not a doctrine of prophetic authority; it is the Lord telling a weeping young man that He has seen his tears and that his mourning is over. Whatever Section 21 establishes about office and order, it establishes it around that tenderness. The God who organizes a kingdom is the same God who counts one man's prayers and notices his grief for Zion. A church led by a prophet is, first, a church led by a God who weeps with His people and then wipes their eyes — and that is the kind of leading the whole book is teaching us to trust.