The Vorlons needed to bring the Minbari and the humans together for the final Shadow war, so they planned ahead. Or behind, I don't know. Their allies Sheridan, Delenn and Sinclair used the Great Machine for its time travel capabilities, and stole Babylon 4 to send a thousand years in the past. The Minbari at that time were fighting the Shadows someplace far from home and they were losing, their base had just been destroyed and their leaders had been killed. Sinclair went into the past with the station, entered a chrysalis formed by a Triluminary, became the great Minbari leader and religious figure Valen, and gave them Babylon 4 to serve as their new base of operations. Two Vorlons appeared openly with Valen to ensure that the Minbari would accept him and the station without question. The Vorlon plan used technology to simulate a miracle, providing the Minbari with a new base, a new leader and a reason to fight. But Sinclair is/was not really a Minbari. At some point the Minbari might figure out that their ancient religious relics, the Triluminaries, are made from pieces of Earthforce tech (Sinclair's comlink), and that the Vorlons transformed a human into the most important Minbari, the leader who brought the three castes together in cooperation and forbade them to kill one another. The Warrior Caste especially seems to regard racial purity as being pretty important, and they'll probably be rather upset by this revelation, that their revered leader was not one of them. They're a surly bunch, so the result will likely be something very violent, like another holy war, to avenge the perversion of their race and religion. No one likes to be played for a fool, after all. The legacies of the Vorlons and Shadows will continue to shape the future of the galaxy.
"One mistake... out of so many."
Lyta Alexander, Thirdspace