On this account (i.e. on account of what has been seen of the SON'S superiority to the angels) we ought (or, we are bound) more abundantly to give heed to the things that we have heard (i.e. the gospel that has been preached to us in the Son), lest at any time (or, lest haply) we let them slip (rather, float past them). The word παραρρυῶμεν (aorist subjunctive from παραρρέω) denotes flowing or floating past anything. The allusion is to the danger, incidental to those to whom the Epistle was addressed, of failing to recognize the transcendent character of the gospel revelation, missing it through inadvertence, drifting away from it.
For if the word that was spoken through angels (i.e. the Law) was made (or, proved) steadfast (i.e. as explained in the next clause, ratified by just visitation of every transgression and disobedience), how shall we (Christians) escape, if we neglect so great salvation? The danger of neglect must be in proportion to the dignity of the revelation. The readers are now further reminded of the manner in which the gospel had been made known to them, and been ratified in their own experience, by way of enhancing the danger of disregarding it. Which (not the simple relative pronoun ἢ, but ἥτις, which denotes always, when so used, some general idea in the antecedent, equivalent to "being such as"), having at the first begun to be spoken through the Lord (opposed to "the word spoken through angels" in the preceding verse. Its beginning was through the Lord himself, i.e. Christ the SON, not through intermediate agency. ὁ κύριος is a special designation of Christ in the New Testament; and, though not in itself proving belief in his divinity, is significant as being constantly used also as a designation of God, and substituted in the LXX. for הוהי. It has a special emphasis here as expressing the majesty of Christ), was confirmed ( ἐβεβαιώθη, answering to ἐγένετο βέβαῖος in the former verse) unto us by them that heard (i.e. by the apostles and others who knew Christ in the flesh). Here the writer ranks himself among those who had not heard Christ himself; his doing which has been considered to afford a presumption against St. Paul having been the writer. For, though not an eyewitness of Christ's ministry, he is in the habit elsewhere of insisting strongly on his having received his "knowledge of the mystery," not from men or through men, but by direct revelation from the ascended Savior (cf. Galatians 1:1, Galatians 1:12). Still, he does not deny elsewhere that for the facts of Christ's history he was indebted to the testimony of others (cf. 1 Corinthians 15:3, etc). It was rather the meaning of the mystery that he had learnt from heaven.