Source identified1
“Angels can fly because they can take themselves lightly.”
Chesterton uses levity as a metaphor against swollen self-importance: the ability to take oneself less heavily becomes part of spiritual freedom rather than evidence that serious responsibilities do not matter.
Source note: Orthodoxy, Chapter VII, “The Eternal Revolution,” Project Gutenberg #130. The sentence belongs to Chesterton’s explicitly Christian contrast between levity and pride; he immediately describes pride as a downward heaviness and self-forgetfulness as something one rises toward.