The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
6. From thence (the souls are led) by him only who belongs to the lightning; the sacred text stating that.
From thence, i.e. after they have come to the lightning they go to the world of Brahman, being led through the worlds of Varuna and the rest by the person, not a man, who follows immediately after the lightning. For that that person leads them is stated in the following passage, 'When they have reached the place of lightning a person, not a man, leads them to the world of Brahman' (Bri. Up. VI, 2, 15). Varuna and the rest, we must understand, favour them either by not hindering or somehow assisting them.--Therefore it is well said that light and so on are the gods who act as conductors.