for the showing, I say, of his righteousness at this present season: that he might himself be just, and the justifier of him that hath faith in Jesus.

He did this to demonstrate His righteousness at the present time, so as to be just and to justify the one who has faith in Jesus.

for the shewing forth of His righteousness in the present time, for His being righteous, and declaring him righteous who is of the faith of Jesus.

for [the] shewing forth of his righteousness in the present time, so that he should be just, and justify him that is of [the] faith of Jesus.