εἴπερ μὴ ψεύσουσι, you will find me helpful,—that is to say, if the gods do not disappoint me. εἴπερ marks the point which must be taken for granted, in order that “ἐρεῖς” (626) should hold good: cp.
. Lys. or. 12 § 48 “εἴπερ ἦν ἀνὴρ ἀγαθός, ἐχρῆν ἄν”, etc. (Cp. Thompson Syntax § 225. 4.)