Elemental imagery
Merton often invokes symbols in their standard interpretive meaning, so darkness represents ignorance and powerlessness, and the light represents love and spiritual enlightenment. Water is often an indication of chaos and disarray. The air is often representative of the spiritual health of a situation. His poems are ripe with elemental imagery: Earth, water, light or fire, and air.
Catholic imagery
Many of Merton's poems contain references to Catholic tradition, which is basically the folklore of the Catholic church (important legendary people are saints for instance), and the Catholic church believes that this tradition is sacred and inspired, just like the Bible. So in addition to biblical allusions, Merton's poetry is also full of beautiful descriptions of Catholic arts and architecture, as well as allusions to Catholic traditions that might even be considered esoteric, due to the vastness of Catholic history.
Descriptions of God's glory
One poetic motif that occurs in many religious poems is the poetic worship of God, the attempt to capture through poetry something about the quality of nature, existence and God that is not easily stated in language. Often Merton describes religious experiences of the divine, intimations of heavenly experiences, or even poetic descriptions of God himself.
Dark symbolism
Merton's poetry is fairly dark in tone, at least this is often the case. This is partially due to Merton's education and deep understanding of the literature of the Bible, a book well-known for its inclusion of horrifying and terror-inducing passages. For instance, Merton describes in "The Dark Morning," the final moments of a prisoner whose cell is flooding, but no one is there to save him and he's going to drown. In "Advice to a Young Prophet," he basically advises to stay paranoid, because the universe is full of terrible, awful evil and chaos beyond imagination. The point of his poetry is not just to inspire fear, although sometimes he does, but rather, he is accepting the darkness to move toward true enlightenment. The darkness is the driving factor in the moral development of the prophet in "Advice to a Young Prophet."
The motif of redemption
Just as some poetry has a twist in the tail, just like sonnets end with turns, Merton's poetry also will turn, usually from darkness or disarray toward enlightenment and deep emotional fulfillment. The question for Merton is whether to participate in the hellish systems of the earth, or to pursue a moral high ground, perhaps one where suffering serves a purpose. This is the question of redemption. He wants the suffering and chaos of the earth to be redeemed by God's glorious power and blessing.
