Our Father in Heaven begat all the spirits that ever were, or ever will be, upon this earth; and they were born spirits in the eternal world. Then the Lord by his power and wisdom organized the mortal tabernacle of man. We were made first spiritual, and afterwards temporal. 1:50.
[W]e are the offspring of God.
I, the Lord God, created all things, of which I have spoken, spiritually, before they were naturally upon the face of the earth.
This is where things get very interesting -- to me, at least.
As it stands, this statement looks like a very straightforward. It looks like the sort of thing you might expect to hear at any General Conference, or in any Sacrament Meeting talk. (However, the scriptures don't actually state that spirits were "begotten" per se -- even D&C 76:24 doesn't state that, although it is often read as if it does. The scriptures do talk about God creating pre-mortal spirits, but does not talk about God begetting them.)
However, when Brigham says that "Our Father in Heaven" begat all of those spirits, he means something substantially different by that term than we usually think. When read in context, in the Journal of Discourses, this statement immediately precedes one of the most direct and affirmative assertions that Adam is our Father and our God, that he was a celestial being before he came to this earth, and that as a celestial being he fathered the spirits of all of the earth's inhabitants, and the mortal body of Jesus Christ.
Brigham's paragraph in this discourse is his first public assertion about (what has since been called) "Adam-God". All sorts of attempts have been made to explain this paragraph, usually revolving around the claim that he was misquoted, or that it was a personal belief, and never put forth by him to the Saints as the revealed word of God. Admittedly, it is true that this paragraph, by itself, can be read (with a little effort) a way that does make it seem to conform to modern-day orthodoxy. However, this quotation does not exist in a vacuum. Many, many more statements by Brigham Young, along with journal entries by his contemporaries, make it clear that he did believe that Adam was God, and that this doctrine was intended as the present-day revelation of God to His people. (For more information about this topic, see especially "The Adam-God Doctrine" by David John Buerger, in Dialogue: A Journal of Mormon Thought, Volume 15, Number 1, Spring 1982, pages 14-58.)
As a result, Brigham Young's quotation, as it appears in Chapter 4, has a very different meaning in its original context than it does today. As it appears in Chapter 4 it is hardly noteworthy, but in its original form it turns out to be one of Brigham's least astonishing assertions about the spiritual Fatherhood of God.