Jamieson, Fausset and Brown on Hebrews 5:2

2. Who can--Greek, "being able"; not pleasing himself (Ro 15:3).

have compassion--Greek, "estimate mildly," "feel leniently," or "moderately towards"; "to make allowance for"; not showing stern rigor save to the obstinate (He 10:28).

ignorant--sins not committed in resistance of light and knowledge, but as Paul's past sin (1Ti 1:13). No sacrifice was appointed for wilful sin committed with a high hand; for such were to be punished with death; all other sins, namely, ignorances and errors, were confessed and expiated with sacrifices by the high priest.

out of the way--not deliberately and altogether wilfully erring, but deluded through the fraud of Satan and their own carnal frailty and thoughtlessness.

infirmity--moral weakness which is sinful, and makes men capable of sin, and so requires to be expiated by sacrifices. This kind of "infirmity" Christ had not; He had the "infirmity" of body whereby He was capable of suffering and death.