Adam Clarke on Hebrews 12:25

See - Βλεπετε· Take heed, that ye refuse not him - the Lord Jesus, the mediator of the new covenant, who now speaketh from heaven, by his Gospel, to the Jews and to the Gentiles, having in his incarnation come down from God.

Him that spake on earth - Moses, who spoke on the part of God to the Hebrews, every transgression of whose word received a just recompense of reward, none being permitted to escape punishment; consequently, if ye turn away from Christ, who speaks to you from heaven, you may expect a much sorer punishment, the offense against God being so much the more heinous, as the privileges slighted are more important and glorious.