Some people find the idea that we are counted righteous by God through believing his promise (Genesis 15:6) difficult to accept. Why should we be thought righteous for believing?

1. We don’t have to prove how it works. Christianity is a revealed religion, not a derived religion. That means we think we know this because it has been revealed, not because we figured it out. In that case, so long as the view isn’t incoherent, and the revelation source is trustworthy, then you are warranted to believe something even if you can’t prove, or even explain it.
2. Faith is the right ground to credit righteousness because it is the first step in being reoriented toward reality and what is good and just, which is embodies in God. Without faith in the true God, then no set of works can rightly orient a person towards reality- moral or otherwise. Without that reality reorienting faith, righteousness is implicitly refused. With faith, by accepting God’s righteousness, God opens us to be credited righteousness and as a vessel in which to develop that righteousness through faith.
3. That’s not the point of this doctrine. The dynamic of promise-faith as the dynamic through which we are made right with God is not what MAKES it work. It works because of other supporting and related truths- imputation, union with Christ and others.
4. How does it work? 2 options:
   1. Use of debt metaphor. When people think of having righteousness credited to a criminal, they often struggle with the idea of a substitute sacrifice. Why is a criminal righteous because an innocent person took their punishment? But it depends on what metaphor of justice you use- criminal or civil. In civil cases, all crimes can be reduced to property and debt since all crime takes something from the person and the community. Justice requires the restoration of what was damaged or destroyed. That debt has to be paid by some kind of restitution. Sin is commonly treated as a kind of moral debt which is paid by Christ’s sacrifice.
   2. Union with Christ. Another way to think about it is that through faith God offers union with himself, namely in the person of Christ. The union of Christ with a believer goes all the way down to being and identity, and the one becomes part with the other- though the metaphysics of that is never explained in detail. This means that the moral status of both are shared. This means that the believing human truly shares in the righteousness of Christ, and is counted righteous “in Him”.

It’s important not to pretend we know more than we do. And it is also important to make clear when we are speculating and when we are explaining something scripture clearly tells us.