In this work, Ralph Waldo Emerson focuses on the importance of avoiding conformity. Choosing individuality over conformity is something he believes is necessary, but it is difficult to do this within the confines of society. Instead of following social norms in terms of what to believe and how to act in certain situations, Emerson advocates that everyone should keep his or her own moral compass and develop beliefs based on that.
However, in this particular quote, he is urging readers to follow God and to allow Him to mold their lives—hence the reference to "noble clay" and "plastic." By using the word "noble," he reinforces his point that being used for God’s will to fulfill the "Almighty effort" is a worthy purpose in life. This implies that God’s will for humanity and society’s will for humanity are often at odds with each other, and Emerson argues that it is highly preferable to follow God than it is to follow society.
Let’s break this down further: if you look at the context of this quote, Emerson is contrasting his idea of what is a desirable attitude with what is not. He states that great men of history were clearly moved by God, and so he urges us to emulate their openness to God’s will. In the same breath, he urges us not to be “cowards fleeing before a revolution.” To sum it up, Emerson believes that great men are open to change, willing to follow God, and unwilling to blindly conform to society.
A link to the text is below.