Never forget, you are a simple human. Weak, simple minded, and powerless individually. You lack the psychic gifts of the Eldar, the strength of the Orks, the adaptation of the Tyranids, the Demonic blessings and curses of the followers of Chaos, the sheer technological prowess of the Necrons, or the innovation of the Tau.
Yet despite all this, you and your fellow brothers and sisters have forged an empire that has conquered an entire galaxy. That has endured horror and sacrifice that has felled "stronger" empires. That has taken the entire weight of the universe' collective hatred for your kind to so much as weaken, let alone fall. We suffer the predilections of every monstrous horror the universe can conjure and imagine and yet still we are here.
Still we endure. Because we are Mankind. And we have what all others lack. A will of steel and fire.
My wondrous brothers and sisters. For the Emperor. For the Imperium. For Mankind.