Final answer:
Longfellow's first poem to gain popularity was “The Psalm of Life,” which was published in 1838 and resonated with readers for its themes of optimism and perseverance.
Step-by-step explanation:
Longfellow's first poem to gain his popularity was “The Psalm of Life”. This poem was published in 1838 and became an instant success, resonating with readers with its themes of optimism, perseverance, and the value of living a purposeful life. It is known for its memorable lines such as 'Lives of great men all remind us / We can make our lives sublime.'