Laudator Temporis Acti

Euripides, Bacchae 501-502 (tr. E.P. Coleridge):

PENTHEUS
Why, where is he? To my eyes he is invisible.
DIONYSUS
He is by my side; thou art a godless man and therefore dost not see him.

ΠΕΝΘΕΥΣ
καὶ ποῦ ᾽στιν; οὐ γὰρ φανερὸς ὄμμασίν γ᾽ ἐμοῖς.
ΔΙΟΝΥΣΟΣ
παρ᾽ ἐμοί· σὺ δ᾽ ἀσεβὴς αὐτὸς ὢν οὐκ εἰσορᾷς.

E.R. Dodds ad loc.:

Vision demands not only an objective condition—the god's presence—but a subjective one—the percipient must himself be in a state of grace.

Valdis Leinieks, The City of Dionysos: A Study of Euripides' Bakchai (Stuttgart: Β.G. Teubner, 1996), pp. 251-252 (footnote omitted):

Although Dionysos is standing in front of him, Pentheus is not able to observe him. Ability to observe requires respect for that which is observed. Pentheus, however, has no respect for anyone or anything. He respects only himself. Hence he is able to observe only what corresponds to his already established perceptions and perceive only what corresponds to his expectations. His expectations, in turn, are based on already established perceptions. Since according to Pentheus' established perception Dionysos does not exist, he is not able to observe him.