A.T. Robertson Commentary Hebrews 9:5

"and above it cherubim of glory overshadowing the mercy-seat; of which things we cannot now speak severally." — Hebrews 9:5 (ASV)

Above it (υπερανω αυτης). Up above, in local sense as in Eph 4:10, with ablative case αυτης (it, the ark).

Cherubim of glory (Χερουβειν δοξης). Hebrew word (dual form), two in number, made of gold (Exodus 25:18–22). They are called ζωα (living creatures) in the LXX (Isaiah 6:2f.; Ezekiel 1:5–10; Ezekiel 10:5–20).

Overshadowing (κατασκιαζοντα). Present active participle of κατασκιαζω, old verb to shadow down on, cover with shade, only here in the N.T.

The mercy seat (το ιλαστηριον). The pinions of the Cherubim spread over the rectangular gold slab on top of the ark termed the mercy seat. Here the adjective ιλαστηριος has to mean mercy seat, the place, not the propitiatory gift or propitiation, as in Ro 3:25 (Deissmann, Bible Studies, pp. 124-35).

Severally (κατα μερος). In detail, distributive use of κατα with μερος (part).