In my previous post quoting Rutherford's argument for why James 2 is not teaching salvation by works, the argument by Rutherford is rather convoluted, thus making it hard for probably most people to grasp. In this post therefore, I would like to simplify and put forward Rutherford's main argument so that it can be better appreciated.
Let us suppose that the passage in James 2 is indeed talking about legal justification. Therefore,
1) From the text of Scripture, verse 21 teaches that Abraham was justified by his work of sacrificing Isaac on the alter. Therefore, verse 21 teaches that the act of sacrificing Isaac caused Abraham to be justified.
2) From verse 23 (quoting Gen. 15:6), it is taught that belief caused Abraham to be justified.
Scenario one shows us that Abraham was justified by works, while scenario two shows us that Abraham was justified by his faith. Both must be true since both are "derived from Scripture" (assuming James 2 is about justification in the legal sense), yet the two contradict each other, an impossibility if one believes in the inspiration of all Scripture.
I may add here that justification being considered in a legal sense means that before it, the person is not justified, and after it he is. Roman Catholics may say that initial justification (by faith) does not preclude subsequent justification (by works), and that is true only if we ignore the fact that we are talking about the legal sense of justification. Being legal, such means that prior to that justification, the person was NOT justified at all. If James 2 teaches that justification happened only after the work (of Abraham sacrificing Isaac), then justification DID not happened after faith as stated in Gen. 15:6, James 2:23.
The only recourse for the logical Roman Catholic would be to deny the legal aspect of justification here. However, that would effectively concede our case — that justification is not to be considered in the legal sense in the passage of James 2.