Jamieson, Fausset and Brown on Hebrews 4:14

14. Seeing then--Having, therefore; resuming He 2:17.

great--as being "the Son of God, higher than the heavens" (He 7:26): the archetype and antitype of the legal high priest.

passed into the heavens--rather, "passed through the heavens," namely, those which come between us and God, the aerial heaven, and that above the latter containing the heavenly bodies, the sun, moon, &c. These heavens were the veil which our High Priest passed through into the heaven of heavens, the immediate presence of God, just as the Levitical high priest passed through the veil into the Holy of Holies. Neither Moses, nor even Joshua, could bring us into this rest, but Jesus, as our Forerunner, already spiritually, and hereafter in actual presence, body, soul, and spirit, brings His people into the heavenly rest.

Jesus--the antitypical Joshua (He 4:8).

hold fast--the opposite of "let slip" (He 2:1); and "fall away" (He 6:6). As the genitive follows, the literally, sense is, "Let us take hold of our profession," that is, of the faith and hope which are subjects of our profession and confession. The accusative follows when the sense is "hold fast" [Tittmann].