Louis Bouyer, The Christian mystery: from pagan myth to Christian mysticism, trans. Illtyd Trethowan (Edinburgh: T & T Clark, 1990), chap. 5, p. 74.
The quotation is (though Bouyer does not say so here) from Iulius Firmicus Maternus, De errore profanarum religionum 22.1, and is translated by Bouyer (or Trethowan) with "if" rather than (as often elsewhere) "since":
θαρρεῖτε μύσται τοῦ θεοῦ σεσωσμένου·
ἔσται γὰρ ὑμῖν ἐκ πόνων σωτηρία.
CSEL 2, ed. C. Halm (1867), p. 112. The variant readings present in CSEL 2 are ΘΑΡΡΙΤΕ ΜΙΣΤΕ and ΕΣΤΕ. But check also the 1982 edition ed. R. Turcan.