Paul here seems to be drawing upon traditional Jewish theology, especially the apocryphal Wisdom of Solomon (13:5, 8-9, RSV). His language is too strikingly similar to this ancient text to be coincidental.
For from the greatness and beauty of created things comes a corresponding perception of their Creator.
Yet again, not even they are to be excused; for if they had the power to know so much that they could investigate the world, how did they fail to find sooner the Lord of these things?
Later, in 9:20, Paul again probably alludes to the Wisdom of Solomon (12:12), when he says, “One of you will say to me, ‘Then why does God still blame us? For who resists his will?’”
For who will say, ‘What hast thou done? Or who will resist thy judgment?
He goes on in 9:21, using the analogy of the potter, where God makes vessels for different reasons, some for noble purposes and some for common use. This analogy, also, has its parallel in the Wisdom of Solomon (15:7).
For when a potter kneads the soft earth and laboriously molds each vessel for our service, he fashions out of the same clay both the vessels that serve clean uses and those for contrary use, making all in like manner; but which shall be the use of each, of these the worker in clay decides.
In 2 Corinthians 5:1, 4, Paul uses the unusual metaphor describing the human body as a perishable tent, once again, echoing language from the Wisdom of Solomon (9:15).
…for a perishable body weighs down the soul, and this earthly tent burdens the thoughtful mind.
None of these allusions demonstrate beyond argument that Paul regarded the Wisdom of Solomon as Scripture, but at the same time, his usage of this intertestamental work does suggest that he valued it and thought it worth referencing. At the very least, no one makes allusions to literary works he hasn’t been reading!