Here's what St. John Chrysostom says about the verse:
In him; to put it by way of corporeal similitude, even as it is impossible to be ignorant of the air which is diffused on every side around us, and is not far from every one of us, nay rather, which is in us. (d) For it was not so that there was a heaven in one place, in another none, nor yet (a heaven) at one time, at another none. So that both at every time and at every bound it was possible to find Him. He so ordered things, that neither by place nor by time were men hindered. For of course even this, if nothing else, of itself was a help to them — that the heaven is in every place, that it stands in all time. (f) See how (he declares) His Providence, and His upholding power (συγκράτησιν); the existence of all things from Him, (from Him) their working (τὸ ἐνεργεἵν), (from Him their preservation) that they perish not. And he does not say, Through Him, but, what was nearer than this, In him.— That poet said nothing equal to this, For we are His offspring. He, however, spoke it of Jupiter, but Paul takes it of the Creator, not meaning the same being as he, God forbid! But meaning what is properly predicated of God: just as he spoke of the altar with reference to Him, not to the being whom they worshipped. As much as to say, For certain things are said and done with reference to this (true God), but you know not that they are with reference to Him. For say, of whom would it be properly said, To an Unknown God? Of the Creator, or of the demon? Manifestly of the Creator: because Him they knew not, but the other they knew. Again, that all things are filled (with the presence)— of God? Or of Jupiter — a wretch of a man, a detestable impostor! But Paul said it not in the same sense as he, God forbid! But with quite a different meaning. For he says we are God's offspring, i.e. God's own, His nearest neighbors as it were.
For lest, when he says, Being the offspring of God Acts 17:29, they should again say, You bring certain strange things to our ears, he produces the poet. He does not say, You ought not to think the Godhead like to gold or silver, ye accursed and execrable: but in more lowly sort he says, We ought not. For what (says he)? God is above this? No, he does not say this either: but for the present this — We ought not to think the Godhead like such, for nothing is so opposite to men. But we do not affirm the Godhead to be like this, for who would say that? Mark how he has introduced the incorporeal (nature of God) when he said, In Him, etc., for the mind, when it surmises body, at the same time implies the notion of distance. (Speaking) to the many he says, We ought not to think the Godhead like gold, or silver, or stone, the shaping of art, for if we are not like to those as regards the soul, much more God (is not like to such). So far, he withdraws them from the notion. But neither is the Godhead, he would say, subjected to any other human conception. For if that which art or thought has found — this is why he says it thus, of art or imagination of man — if that, then, which human art or thought has found, is God, then even in the stone (is) God's essence.— How comes it then, if in Him we live, that we do not find Him? The charge is twofold, both that they did not find Him, and that they found such as these. The (human) understanding in itself is not at all to be relied upon.— But when he has agitated their soul by showing them to be without excuse, see what he says: The times of ignorance God overlooked, but now commands all men everywhere to repent. Acts 17:30 What then? Are none of these men to be punished? None of them that are willing to repent. He says it of these men, not of the departed, but of them whom He commands to repent. He does not call you to account, he would say. He does not say, Took no notice (παρεἵδεν); does not say, Permitted: but, You were ignorant. Overlooked, i.e. does not demand punishment as of men that deserve punishment. You were ignorant. And he does not say, You wilfully did evil; but this he showed by what he said above. — All men everywhere to repent: again he hints at the whole world. Observe how he takes them off from the parcel deities! Because He has appointed a day, in the which He will judge the world in righteousness by that Man whom He has ordained, whereof He has given assurance to all men, in that He raised Him from the dead. Acts 17:31 Observe how he again declares the Passion. Observe the terror again: for, that the judgment is true, is clear from the raising Him up: for it is alleged in proof of that. That all he has been saying is true, is clear from the fact that He rose again. For He did give this assurance to all men, His rising from the dead: this (i.e. judgment), also is henceforth certain.