The Greek particle mēn is a strong affirmative particle, related to the oath formula and used to introduce solemn assurances. It appears in Hebrews 6:14 in God's oath to Abraham: 'Mēn blessing I will bless you, and mēn multiplying I will multiply you' — a rendering of the Hebrew oath formula emphasizing absolute certainty. The double use (with the infinitive) conveys the unbreakable nature of the divine promise.
In Hebrews 6:13–14, the author shows that God swore by Himself — since there was no one greater by whom to swear — using the particle mēn (surely/certainly). The theological point is the absolute reliability of the divine promise to Abraham and to all his heirs in Christ. When God says 'mēn,' the promise is backed by His own character and cannot fail. This is the foundation of Christian assurance: not our grip on God but God's oath-bound word holding us. Hebrews 6:19 calls this hope 'an anchor for the soul, firm and secure.'