Ye have net yet resisted unto blood, striving against sin. Here (as in 1 Corinthians 9:26) there is a transition of thought from a race to a combat. Your trials have not yet reached the point of dying in the good fight of faith, as has been the case with some of your brethren before you, who have followed their Leader to the end (of. Hebrews 13:7).
And ye have forgotten (or, have ye forgotten?) the exhortation which speaketh unto you (more correctly, discourses, or reasons, with you; i.e. in the way of fatherly remonstrance) as unto children, My son, etc. This verse introduces a further motive for persevering under prolonged trial, viz. our being assured in Holy Writ of its beneficial purpose as discipline. The quotation is from Proverbs 3:11, Proverbs 3:12, as it is in the LXX. We observe that the word "faint" ( ἐκλύου) is the same as was used in Proverbs 3:3. In the seventh and following verses this scriptural admonition is applied and commented on.
For chastening ye endure; i.e. It is for chastening that ye endure. The reading εἰς παιδείαν ὑπομένετε, supported by almost the whole weight of manuscripts (including all the uncials that contain the text), of ancient versions, and commentators (Theophylact being the only certain exception), is decidedly to be accepted instead of the εἰ παιδείαν ὑπομένετε (equivalent to "if ye endure chastening") of the Textus Receptus. Moreover, it is required for the sense of the passage in regard to the proper meaning of the verb ὑπομένετε ("endure"), which is to "submit to," or "endure patiently," not simply "to undergo." For to say, "if ye endure chastisement patiently, God dealeth with you as sons," has no meaning; our being treated as sons depends, not on the way we take our chastisement, but on our being chastised at all. The use of the preposition εἰς to express purpose is common in this Epistle (cf. Hebrews 1:14, εἰς διακονίαν: Hebrews 3:5, εἰς μαρτύριον: Hebrews 4:16, εἰς βοήθειαν: Hebrews 6:16, εἰς βεβαίωσιν): and the essential sense of παιδεία is discipline or education. The drift is the same, whether we take ὑπομένετε as an indicative or an imperative. Thus the next clause of the verse follows suitably: God dealeth with you as with sons; for what son is there (or, who is a son) whom his father chasteneth not? But if ye be without chastening, whereof all (i.e. all God's children, with reference to Hebrews 11:1-40) have been made partakers, then are ye bastards, and not sons (ye are not your father's real children whom he cares for as such).