Albert Barnes Commentary Hebrews 10:28

"A man that hath set at nought Moses law dieth without compassion on [the word of] two or three witnesses:" — Hebrews 10:28 (ASV)

He that despised Moses' law. This refers to the apostate from the religion of Moses. It does not mean that in all cases the offender against the law of Moses died without mercy, but only where offenses were punishable with death. The apostle probably had in mind particularly the case of apostasy from the Jewish religion.

The subject of apostasy from the Christian religion is particularly under discussion here, and it was natural to illustrate this by referencing a similar case under the law of Moses. The law regarding apostates from the Jewish religion was definite: there was no reprieve (Deuteronomy 13:6–10).

Died without mercy. This means there was no provision for pardon.

Under two or three witnesses. It was the settled law among the Hebrews that in all cases involving capital punishment, two or three witnesses were necessary. This means no one was to be executed unless at least two persons bore testimony, and it was considered important, if possible, for three witnesses to concur in the statement. The object was the security of the accused person if innocent. The principle in the law was that it was presumed two or three persons would be much less likely to conspire to give false testimony than one person would be, and that two or three would not likely be deceived about a fact they had observed.