Final answer:
Abraham Lincoln referred to slavery as 'the moral wrongness of slavery' in his first inaugural address, highlighting his condemnation of the institution as unethical.
Step-by-step explanation:
The phrase used by Abraham Lincoln to express his opinion of the institution of slavery in his first inaugural address is "the moral wrongness of slavery." This statement encapsulates the notion that slavery is fundamentally unethical and immoral. Moreover, during that era, various historical figures and texts grappled with slavery from many angles, some denouncing it as a moral evil while others, particularly pro-slavery factions, sought to justify its practice.