This seems to be from Kenneth Cavander’s translation of Euripides’ The Bacchae, which was published in 1968 under the title The Bacchae of Euripides (University of Nebraska Press). It was performed at Yale University in 1969:
In the spring of 1969, the Repertory Theatre of the Yale School of Drama mounted a production [of The Bacchae] using the Kenneth Cavander translation, directed by André Gregory.
Karelisa Hartigan (1995). Greek Tragedy on the American Stage : Ancient Drama in the Commercial Theater, 1882–1994, p. 83. Westport: Greenwood Press.
A report on the production in Life magazine included the quotation:
In choosing to produce The Bacchae at Yale, Dean Robert Brustein says he was inspired by a harrowing photograph of hippies during their recent march on Washington when they confronted troops with bayonets at the Pentagon Building. […]
The timeliness of Euripides’ play extends beyond its theatrical interests into the realm of modern morals. It makes its point in the simplest possible words, spoken by a messenger who is stunned by the Dionysian horrors he has just witnessed: “The best and safest thing is to keep a balance in your life, acknowledge the great powers around us and in us. If you can do that, and live that way, you are a wise man.”
Tom Prideaux (1969). ‘A cry from the past for an artistic conscience’. In Life 66:13 (April 4, 1969), pp. 32–34.
The quotation is a loose translation of the lines below, which follow immediately upon the messenger’s description of Pentheus being ripped apart by his mother Agave and the Bacchantes:
τὸ σωφρονεῖν δὲ καὶ σέβειν τὰ τῶν θεῶν
κάλλιστον: οἶμαι δ᾽ αὐτὸ καὶ σοφώτατον
θνητοῖσιν εἶναι κτῆμα τοῖσι χρωμένοις.
Soundness of mind and reverence for the affairs of the gods is best; and this, I think, is the wisest possession for those mortals who adopt it.
Euripides. The Bacchae 1150–1152. Translated by T. A. Buckley (1850). Perseus Digital Library.