Know ye therefore ( γινώσκετε ἄρα); or, ye perceive then. Critics are divided between the two renderings, the imperative and the indicative, both here and Matthew 24:43; 1 John 2:29. In Luke 10:11 and Hebrews 13:23 γινώσκετε is certainly imperative.
The categorical imperative seems of the two the more suited to the apostle's impetuous temperament. The verb γινώσκω, like the Latin nosco, properly denotes "to come to know," "learn," "perceive," "get apprised;" ἔγνωκα or ἔγνων, like now, having more properly the sense of "knowing."
But this distinction does not always hold, as e.g. Romans 7:1. That they which are of faith ( ὅτι οἱ ἐκ πίστεως); that the men of faith; that is, who derive their position from faith, belong to faith, are above all things characterized by faith.
Compare the expressions, τοῖς ἐξ ἐριθείας, "the men of factiousness, i.e. "factions men" (Romans 2:8); τὸ ἐκ πίστεως ἰησοῦ, "the man of faith in Jesus," taking his stand thereupon (Romans 3:26).
Closely affine to this usage of the preposition, if not quite the same, is, ὁ ὢν ἐκ τῆς ἀληθείας, "that is of the truth" (John 18:37); οἱ ἐκ νόμου, "they which are of the Law" (Romans 4:14); ὅσοι ἐξ ἔργων νόμου εἰσίν, (Romans 7:10 of this chapter).
The same are the children of Abraham ( οὗτοί εἰσιν υἱοὶ ἀβραάμ); these are sons of Abraham. The form of expression is precisely the same as in Romans 8:14, "As many as are led by the Spirit of God ( οὗτοί εἰσιν υἱοὶ θεού) these are sons of God."
In both cases the absence of the article before viol suggests the feeling that the apostle is simply stating a predicate of the class before defined, but not now affirming that this predicate is confined to that class, although, again in each case, he knew that it was so confined.
Just here, what he is concerned to affirm is that the possession of faith is a complete and sufficient qualification for sonship to Abraham. There is, perhaps, a polemical reference to the teaching of certain in Galatia, that, to be sons of Abraham or interested in God's covenant with his people, it behoved men to be circumcised and to observe the ceremonial Law.
This error would be satisfactorily met by the affirmation of the present verse, that the being believers, simply this, constitutes men sons of Abraham. In the tenth verse the apostle goes further, aggressively denying to those who "were of the works of the Law" the possession at all of Abrahamic privilege.
The class, "men of faith," did in fact include Jewish believers as well as Gentile; but just hero, as seems probable from what is said in the next verse, the apostle has in view Gentile believers only.
The writer's thoughts are hovering round that promise of God ("So shall thy seed be") which had been on that particular occasion the object of Abraham's faith. That this was the case we may infer from his citation of the words in Romans 4:18, the explanation of which had been prepared for by him in what he has said before in Romans 4:16, "To the end that the promise may be sure to all the seed: not to that only which is of the Law, but to that also which is of the faith of Abraham, who is the father of us all."
It was this that led him to speak of being sons of Abraham. This train of thought is pursued further in the next two verses.