In the chapter on justification in 18 Words: The Most Important Words You Will Ever Know, J. I. Packer explains what Paul means in Romans 4 in one of the phrases that some interpret incorrectly and use to support the teaching that Christ’s righteousness is not imputed to the sinner in justification, but rather, it is an individual’s own faith that is counted as their righteousness (See here, for example.).
The rendering which declares Abraham’s faith to have been ‘reckoned’ or ‘counted as righteousness’ (vv. 3, 5, 9, 22), though found in RSV, NIV, NEB and most modern versions, is no good…. ‘As’ represents the Greek preposition eis, meaning ‘towards’ or ‘with a view to’ in a wide range of contexts, and ‘for rightousness’ (KJV, RV) was a much better way to translate it, although ‘reckon’ and ‘count’ are no doubt improvements on the older word ‘impute’. Paul is not saying here that faith is our righteousness, but that we are justified through believing. Certainly, faith is the occasion and means of our justification, but Christ’s obedience (5:19), His righteousness…(v. 18), His propitiation for our sins (3:25…), is its ground.
John Piper agrees that eis should be translated “for” or “unto” rather than “as” (See Counted Righteous in Christ, footnote on page 62.) in Romans 4:3, 5, 9, 22. He also explains that
in Paul’s mind, “faith being credited for righteousness” is shorthand for faith being the way an external righteousness is received as credited to us by God—namely, not by working but by trusting him who justifies the ungodly[.] Paul’s conceptual framework for imputation in verses 4 and 5 would, therefore, not be God’s crediting something we have to be righteousness, but God’s crediting a righteousness we don’t have to be ours by grace through faith.
I’ll admit I’ve had a hard time explaining exactly why this phrase means what it means and these two quotes have helped.