Emerson is speaking to future ministers. In this speech, he tries to implore them to be less focused on Church doctrine and more focused on genuine spiritual connection to God and nature.
Emerson begins the address with meditations on the abstract conditions of things like Beauty, Truth, and Justice and how they are intertwined with Nature. This is an elucidation of his Transcendentalism, a combination of self-reliance and the connection between the self (and humanity as a whole) and nature. Good is a positive force in nature; evil is a lack of this force. Thus, to be a good, willing individual, one must be in tune to such a force in nature. "Whilst a man seeks good ends, he is strong by the whole strength of nature." This is all a spiritual connection.
Emerson goes on to suggest that this spiritual connection is compromised when it is governed by an institution; namely, organized religion (in this case, it is a particular attack against the Unitarian Church of which Emerson used to be Minister).
Emerson encouraged self-reliance in everyone. He believed that each individual could (transcendentally) connect with nature, the force of goodness, God, and so on - directly. Therefore, an intermediary will only interfere with these spiritual connections. Following the supreme example of Jesus (as God innate and/or intuitive in man), Emerson contends that all individuals can/should aspire to this intuitive connection with God and nature. Therefore, this connection is not something taught by an institution or doctrine. It is something arrived at individually.
Here is where Emerson stirred up controversy. He lists the errors of historical Christianity. The first is that Christian doctrine has led o the worship of Jesus the person rather than Jesus the soul. Focusing on persons leads to imitation (hardly self-reliant); whereas focusing on Jesus as a soul, the individual is open to intuiting Jesus' pure teachings/soul. The second error stems from the Church's focus on Jesus as a (historical) person again. This implies that Jesus' revelations to the world happened long ago and are done; leaving us only to imitate it or follow Church doctrine regarding his revelations/teachings. Instead, Emerson argues that such a revelation should be an ongoing experience to all generations. Thus, it is more to experience revelation than it is to merely imitate the Church's representation of it.
Emerson continues his criticism of Christian doctrine, saying that it has been usurped by "formalists" - those preoccupied with the formal aspects of worship and ritual. What is needed, he argues, are ministers who preach soulful connection to God and Nature. Emerson's general argument is to focus more on the soul than the institution of the Church.