It was to show his righteousness at the present time, so that he might be just and the justifier of the one who has faith in Jesus.
26. For a demonstration, There is a different preposition used here, πρὸς, while εἰς is found in the preceding verse. The meaning seems to be the same, for both prepositions are used to designate the design, end, or object of any thing.
From all this Paul infers that it is in vain to look for justification by the works of the law, and that it is to be had only by faith, which is the point he has been all along proving, from Rom 1:17, and which he lays down (Rom 3:28) as the summary of his discourse, with a quod erat demonstrandum - which was to be demonstrated.
Commenting on Romans 3:19-31
Seeing it is one God,.... God is one in nature and essence, though there are three persons in the Godhead, Father, Son, and Holy Ghost; whence it appears, that he that is the God of the Jews, is also the God of the Gentiles, or there would be more gods than one; and that these are justified in one and the same manner, or God...