He was breaking down the logic correctly...
Works was a man made system for others to follow to secure eternal life.
But, one sole work of believing is not related to such a system.
Believing is not to be confused with a system "works" as it was being used in that day.
Works as prescribed by those whop dared to give them were not having any consistent thing to do.
One religious hypocrite could name several things, while another, would speak of other things to do to be saved.
He then brought them out and asked, “Sirs, what must I do to be saved?”
They replied, “Believe in the Lord Jesus, and you will be saved—you and your household.” Acts 16:31
That was a single work. The only one that was to bring salvation.
There is no merit in believing.
The one being believed in holds the merit in regard to Jesus Christ.
If I take something to cure an illness, and it does?
It is what was taken that caused the cure.
That substance holds the merit!
Not the one taking it.
Willing to take it did not cure him.
What was taken, did.