Paul vs James

Some teachers may think Paul believed justification was completed in Genesis 15:6 while James saw it in Genesis 22:16. Trying in some way to split the two.

Paul did the same thing James did by using the same terminology he used in Genesis 15:6 in relation to Genesis 17 & 18 where Abraham conceived with Sarah. Some years after Genesis 15. 

Genesis 15:6 - Abraham believed God and it was counted to him for righteousness  (Gen 15)

Romans 4:19 - Abraham believed God and it was counted to him for righteousness  (Gen 17)

James 2:23 - Abraham believed God and it was counted to him for righteousness  (Gen 22)

The term justified is only used concerning Genesis 22, by James, the word justified is not used in Genesis 15:6..

Only believed and accounted for righteousness.

Keep in mind Abraham was seventy five in Genesis 12, approximately 85 when Ishmael was conceived  after Gen 15, approximately 99 when circumcision was given, and about 100 when Paul says his faith was accounted again at the conception of Isaac.

Rom 4:19  And being not weak in faith, he considered not his own body now dead, when he was about an hundred years old, neither yet the deadness of Sara's womb: 

Rom 4:20  He staggered not at the promise of God through unbelief; but was strong in faith, giving glory to God; 

Rom 4:21  And being fully persuaded that, what he had promised, he was able also to perform. 

Rom 4:22  And therefore it was imputed to him for righteousness. 

We see here that Paul did not see Genesis 15:6 as the end all for imputing faith, since he was 75 in Genesis 12 and 100 in Genesis 18, but Abraham's belief and actions in Genesis 17 were cause for faith being imputed for righteousness.

We shouldn't see James' use concerning Genesis 22 as anything out of the ordinary. Paul did the same. What it reveals is an on going process of activity. It shows that imputing faith can be progressive over several years, culminating in our death as we live out the faith.

Gal 5:5  For we through the Spirit wait for the hope of righteousness by faith. 

We shouldn't concede that our ultimate righteousness is completed as we still hope for it. We will be raised incorruptible.

Paul also stating in Romans that our ultimate justification is ongoing.

Rom 5:21  That as sin hath reigned unto death, even so might grace reign through righteousness unto eternal life by Jesus Christ our Lord. 

Of all the instances where faith is reckoned to Abraham, only Genesis 22 specifically ties it to a current justification. He was justified through faith as it was progressively accounted, finalized in Genesis 22 it seems.

So, we can have faith accounted without justification being finalized.

After accounting faith he still had to be justified in Genesis 22 by works perfecting faith.

Also remember the word believe in Genesis means long continuance or faithfulness. It isn't a one moment belief.