Hebrews 12:14

follow peace with all men--with the brethren especially (Rom 14:19), that so the "lame" among them be not "turned out of the way" (Heb 12:13), and that no one of them "fail of the grace of God" (Heb 12:15). holiness--a distinct Greek word from God's "holiness" (Heb 12:10). Translate here "sanctification." His is absolute holiness: our part is to put on His holiness, becoming "holy as He is holy," by sanctification. While "following peace with all men," we are not so to seek to please them, as to make God's will and our sanctification a secondary object; this latter must be our first aim. (Gal 1:10). without which--Greek, "apart from which." no man shall see the Lord--no man as a son; in heavenly glory (Rev 22:3-4). In the East, none but the greatest favorites are admitted to the honor of seeing the king (compare Sa2 14:24). The Lord being pure and holy, none but the pure and holy shall see Him (Mat 5:8). Without holiness in them, they could not enjoy Him who is holiness itself (Zac 14:20). The connection of purity with seeing the Lord, appears in Jo1 3:2-3; Eph 5:5. Contrast Heb 12:16 (compare Th1 4:3). In Mat 24:30; Rev 1:7, it is said that all shall see the Lord; but, that shall be as a Judge, not as their lasting portion and God, which is meant here.

— Jamieson-Fausset-Brown Commentary (public domain)