For it is written in the law about Abraham's two sons [Gen. 16]; one [was] by his slave woman [Hagar] and the other by the free woman, [his wife, Sarah].

But, just as it was then --- the son born in the ordinary way [i.e., Ishmael] persecuting the one born according to the Holy Spirit's [leading, i.e., Isaac] --- so it is now [i.e., Jews urging compliance with the requirements of the law of Moses persecuting those who refuse to return to dependance on that law for salvation. See 3:1-3].

still we know that a person is not justified [i.e., made right with God] by [perfect obedience to] the law of Moses, but rather through faith in [the person and work of] Christ. [Knowing this] we have trusted in Christ Jesus [to save us], so that we might be made right with God by trusting in Him and not by [our compliance with] the requirements of the law of Moses. Because by such law-compliance no one can be made right with God.

Is the law of Moses contrary to the promises of God? Certainly not! For if there had been a law given which could provide [never ending] life, then a person could be considered right with God by [obeying the requirements of such] a law.