In the first line, the persona uses the verb "were." What does this tell us about those whom he considers truly great?

At the beginning, the speaker is talking about people who once "were" alive. This is a poem about human potential (past or future), but the speaker does focus on the past and those who "were truly great."
But note that this emphasis on the past is more complex than a simple acknowledgment of the great ones in history. In the first stanza, the speaker talks about how the great ones remember the soul's history and how this occurs through "corridors of light" with time measured in "suns." These are Platonic ideas. Plato said that absolute truth is represented by the sun: the eternal light. He also said that a person is born and, in order to achieve great knowledge, he/she must remember what his/her eternal soul has forgotten in the process of being born into the world. This is called anamnesis and it means that a person must remember what he knew prior to being born.
Given the context of the rest of the poem, this idea of remembering relates to the history of human knowledge and the more abstract idea of spiritual humanity. With the former, there is the notion that a great thinker can learn, relearn, or intuit human ideas of the past. With the latter, there is the notion that a great thinker is spiritually connected to great knowledge in an abstract or universal way. So, these materialistic and spiritual ideas of remembering and relearning imply that the great thinkers are always already connected to the past in different ways. This is why "were" is in the title and in the first line. Any great thinker in the future will likewise understand these material and spiritual connections to the past.