Fear had sensed the power of the divine even before he turned back. He just didn't believe it. He knew the remaining divines were all here. When he saw Master is the one behind him, it was as if proving his skepticism. No way this outworlder was a divine being. Master probably possessed something that exuded divine power, like those divine treasures Master had pursued so passionately.
That's why he failed to react even when he sensed an energy spike from Master. Combined with the fact that his energy was still merged with Wrath and Greed in locking down Lucifer, he failed to react when Master shoved a hand toward him.
A blast erupted following that shove. That blast was so powerful it threw Fear forward. Due to the blast, Fear's energy link with Wrath and Greed was severed. A violent backlash of energy struck the three. The obstructive veil covering Lucifer was gone.