‘And Paul stood in the midst of the Areopagus, and said, “You men of Athens, in all things, I perceive that you are very religious. For as I passed along, and observed the objects of your worship, I found also an altar with this inscription, TO AN UNKNOWN GOD. What therefore you worship in ignorance, this I set forth to you.'
That we have here only the bare bones of Paul's words is obvious. He would hardly have been foolish enough to seek to dismiss the Areopagites with so few words. But we have no reason at all to deny that the ideas are Paul's. Rather we must see Luke and his source as summarising the gist of what he said. Silas may well have been present at this speech and have conveyed its content to Luke when he went back to Macedonia. or Luke may have obtained the details from Dionysius the Areopagite.
Paul's speech reveals that he had some knowledge of the teachings of these men and of the teachers whose writings they revered. He had been brought up in the University city of Tarsus. And he wanted to make quite clear that the message he brought was not something totally new, it was not ‘a novelty' to be cursorily listened to and then discarded, but was related to aspects of things that they acknowledged but admitted themselves that they did not fully know and understand. He was speaking of things which they had admitted to being relevant, but which they agreed were not within their ken, for they had altars to ‘unknown gods'. He wanted also to find some common ground, and brought up aspects of the knowledge of God which are known to all men. Thus he begins by referring to what he has seen around them.
When speaking to the Jews he had always begun with their history which was the source of their religion (and no doubt had done with the Jews here). But here he has to begin with the basics of religion, while recognising that he was facing both idol worshippers and philosophers. He points out that he has noticed how ‘very religious' they are. We can compare the use of the same word in Acts 25:19 where it refers respectfully to a religion which the speaker does not wish to deride (the man he was speaking to believed in it and he would not want to offend him) but which did not apply to himself. Thus while it can mean ‘superstitious', it would be taken by his hearers rather as complimentary. They saw themselves as ‘religious' men.
He points out that he has noticed many altars, and many shrines. Athens was full of altars and idols of all kinds and were proud of them. They proliferated. And as he had walked about he had noticed that they had an altar there with the inscription, ‘to an unknown god'. (Being unknown it could have had no image). Well, that is why he was there, to bring to their knowledge this God Whom some of them worshipped and whom they admitted was as yet unknown to them. It was after all an open admission by Athens that there was a void in their religion, and it was one that he wanted to fill.
Altars ‘to unknown gods' seem to have been known in the ancient world because men sought to cover their admitted ignorance of the ways of the gods by making such offerings to ‘unknown gods' in order to cover any gods they may have overlooked and not have covered in their normal sacrifices, lest they be thought to be failing to offer due reverence to some god of whom they were not aware and then find themselves later being dealt with accordingly. For it was their view that the failure to pay all gods due reverence, even unknown ones, might be disastrous. They were ‘catch-all' altars, ensuring that they did not slight gods of whom they were not aware.
Mention is made of such altars in or near Athens by Philostratus and Pausanius, and an altar has been discovered at Pergamum possibly inscribed ‘to the unknown deities' (or it may have been ‘to the holy deities', but either way it was anonymous and imageless). Alternately in a city of altars like Athens it may be that there was an altar which had become buried, and had then been rediscovered, which may then have been dedicated to its ‘unknown God'. Or it may have been one that had been built or rededicated to appease the dead when an ancient burial site was discovered, the god of the deceased not being known. It would be a typically Athenian gesture.
Whether ‘to an unknown god' (singular) was Paul's interpretation of such altars, in view of the fact that he wanted to emphasise that he only spoke of one God, or whether he had actually seen that exact wording on an altar for one of the reasons mentioned, we have no way of telling. But either way his approach emphasised the oneness of the God of Whom he spoke, and their own self-admitted ignorance, and there is no reason for suggesting that he is inventing having seen such an inscription. He refers to it because he did not wish them to think that he was bringing to Athens something totally novel. He was, he said, in his preaching filling in the gap that they admitted that they had in their knowledge.
The use of this idea of the ‘unknown god' would to some extent appeal to all his listeners. The idol worshippers would be drawn in by the fact that they did offer such worship, or at a minimum allowed it, the Epicureans because they saw all gods as unknowable, and the Stoics because they may well have agreed that the eternal Reason was ‘unknown' and that they were seeking to know it. Thus all in one way or another believed in an ‘unknown God'.