To understand the topic and purpose of this address, it is important first to understand that Emerson was addressing students at a divinity school, a school at which students were being prepared to become ministers, and a school at which they were being prepared for their futures in ways consistent with Christian tradition. Emerson's speech was intended to effectuate a change in the ministry, based upon two criticisms he had of Christianity in its then form, so, in many ways, his purpose was subversive.
He begins with a celebration of the beauty of the world, and implies that the beauty and mechanics of nature are not inconsistent with God or with goodness:
The child amidst his baubles, is learning the action of light, motion, gravity, muscular force; and in the game of human life, love, fear, justice, appetite, man and God, interact (para. 4).
He develops this idea more fully and then, before circling back to this at the very end, he offers his criticisms.
His first criticism is with the celebration of the person of Jesus and all of the rituals associated with him, as he says, "It has dwelt, it dwells, with noxious exaggeration about the person of Jesus" (para. 15). This worship of one person, he says, interferes with the holiness in all of us, and Christianity has frozen into a hierarchy more concerned with ritual than with the goodness of man.
His second criticism is a consequence of the first. Because Christianity has become "frozen," we act as though it were all in the past and not a living religion that has contemporary meaning to all. He says, "Men have come to speak of the revelation as somewhat long ago given and done, as if God were dead" (para. 19.)
Flowing from these criticisms is Emerson's exhortation to the students to go forth and "dare to love God without mediator or veil" (para. 30), in other words to not let the hierarchy and tradition get in the way of goodness, God, and the soul of the individual.
He returns at the very end to the theme with which he opens, looking for
...the new Teacher, that...shall see the world to be the mirror of the soul; shall see the identity of the law of gravitation with purity of hear; and shall show that the Ought, that Duty, is one thing with Science, with Beauty, and with Joy (para. 35).
It is important to understand that Emerson was part of the Transcendentalist movement, which focused on individualism, a love of nature, and a dislike of ritual that interfered with either. This is a classic Transcendentalist address, to persuade the next generation of ministers to not allow their ministry to be diluted by a church that interfered with the soul and goodness of the individual or with a celebration of knowledge, beauty, nature, and joy, through its "frozen" worship of what should be a living God. It should be noted that this is a very brief summary of the speech and its development of its ideas, and it is well worth reading every single word of it!