Summary: Abraham was not justified by works... nor by the law.
ABRAHAM BELIEVED GOD
Righteousness is not something which we can accomplish, but rather comes as a gift from God through faith in Jesus Christ (Romans 3:21-22; Romans 3:26). The Apostle Paul illustrates this by precedent: “Abraham believed God, and it was reckoned to him for righteousness” (Romans 4:3; cf. Genesis 15:6). The Apostle reinforces this argument by equating ‘the man to whom God reckons righteousness without works’ (Romans 4:6) with David’s ‘blessed man’ in Psalm 32:1-2, ‘to whom in no wise will the Lord reckon sin’ (Romans 4:8).
Paul refers to Abraham as “our father” (Romans 4:1). Later the Apostle will refer to Abraham as the father of all who follow Abraham’s faith (Romans 4:11-12), throughout all nations (Romans 4:16-17). Paul’s argument is that Abraham was not justified (made righteous) by works, otherwise he would have grounds of boasting - and who can boast before God? (Romans 4:2).
The language of reckoning relates to accountancy. “Abraham believed God, and it was reckoned/ credited/ accounted to him as righteousness” (Romans 4:3). The same verb is used by Paul when he asks that any debt owed by Onesimus be put to the Apostle's own account (Philemon 1:18).
Paul contrasts two types of income. First, there are wages, which are “not because of grace, but because of debt” (Romans 4:4). Second, there is the gift of righteousness - which is freely given to those who believe in the God who justifies the ungodly (Romans 4:5).
This does not, however, turn ‘believing God’ into a substitute for works. The focus is not on our ability to believe - our faith, if you will - but on the one in whom we place our trust: i.e. “the one who justifies the ungodly” (Romans 4:5). Faith is not meritorious, but is the channel through which we receive this gift of God’s grace.
If Abraham was not justified by works (which he was not), then it stands to reason that he was not justified by law. “The promise… was not through the law, but through the righteousness of faith” (Romans 4:13). The promise of Abraham’s numerous offspring is the context in which Abraham’s faith was first expressed (Genesis 15:5-6).
Yet the promise also relates to a land (Genesis 13:15). In fact, the whole world is on offer for Jesus’ meek followers (Matthew 5:5; cf. 1 Corinthians 3:22). Thus, it is in the seed of Abraham (Galatians 3:16) that all the nations of the world are being blessed (cf. Genesis 12:3; Genesis 18:18; Genesis 22:18).
The argument continues that, if the inheritance belongs to those who supposedly keep the law, then faith has been emptied, and the promise has been rendered ineffective (Romans 4:14; cf. Galatians 3:18). Law brings wrath, but where there is no law there is no breach of the law, and therefore no wrath (Romans 4:15; cf. Romans 7:7-8). Salvation, covenant righteousness, justification - whatever we want to call it - has its source in God, and is received only by faith (Romans 4:16; cf. Ephesians 2:8-9).