The following is from Matthew Henry:
He proceeds to call them all to repent of their idolatries, and to turn from them. This is the practical part of Paul's sermon before the university having declared God to them (Acts 17:23), he properly presses upon them repentance towards God, and would also have taught them faith towards our Lord Jesus Christ, if they had had the patience to hear him. Having shown them the absurdity of their worshipping other gods, he persuades them to go on no longer in that foolish way of worship, but to return from it to the living and true God. Observe,
The conduct of God towards the Gentile world before the gospel came among them: The times of this ignorance God winked at. They were times of great ignorance. Human learning flourished more than ever in the Gentile world just before Christ's time but in the things of God they were grossly ignorant. Those are ignorant indeed who either know not God or worship him ignorantly idolatry was owing to ignorance.
These times of ignorance God winked at. Understand it, As an act of divine justice. God despised or neglected these times of ignorance, and did not send them his gospel, as now he does. It was very provoking to him to see his glory thus given to another and he detested and hated these times. So some take it. Or rather, As an act of divine patience and forbearance. He winked at these times he did not restrain them from these idolatries by sending prophets to them, as he did to Israel he did not punish them in their idolatries, as he did Israel but gave them the gifts of his providence, Acts 14:16,17. These things thou hast done, and I kept silence, Psalm 50:21. He did not give them such calls and motives to repentance as he does now. He let them alone. Because they did not improve the light they had, but were willingly ignorant, he did not send them greater lights. Or, he was not quick and severe with them, but was long-suffering towards them, because they did it ignorantly, 1 Timothy 1:13.
The charge God gave to the Gentile world by the gospel, which he now sent among them: He now commandeth all men every where to repent--to change their mind and their way, to be ashamed of their folly and to act more wisely, to break off the worship of idols and bind themselves to the worship of the true God. Nay, it is to turn with sorrow and shame from every sin, and with cheerfulness and resolution to every duty.
This is God's command. It had been a great favour if he had only told us that there was room left for repentance, and we might be admitted to it but he goes further, he interposes his own authority for our good, and has made that our duty which is our privilege.
It is his command to all men, every where,--to men, and not to angels, that need it not,--to men, and not to devils, that are excluded the benefit of it,--to all men in all places all men have made work for repentance, and have cause enough to repent, and all men are invited to repent, and shall have the benefit of it. The apostles are commissioned to preach this every where. The prophets were sent to command the Jews to repent but the apostles were sent to preach repentance and remission of sins to all nations.
Now in gospel times it is more earnestly commanded, because more encouraged than it had been formerly. Now the way of remission is more opened than it had been, and the promise more fully confirmed and therefore now he expects we should all repent. "Now repent now at length, now in time, repent for you have too long gone on in sin. Now in time repent, for it will be too late shortly."
The great reason to enforce this command, taken from the judgment to come. God commands us to repent, because he hath appointed a day in which he will judge the world in righteousness (Acts 17:31), and has now under the gospel made a clearer discovery of a state of retribution in the other world than ever before.
Observe, The God that made the world will judge it he that gave the children of men their being and faculties will call them to an account for the use they have made of them, and recompense them accordingly, whether the body served the soul in serving God or the soul was a drudge to the body in making provision for the flesh and every man shall receive according to the things done in the body, 2 Corinthians 5:10. The God that now governs the world will judge it, will reward the faithful friends of his government and punish the rebels.
There is a day appointed for this general review of all that men have done in time, and a final determination of their state for eternity. The day is fixed in the counsel of God, and cannot be altered but it is his there, and cannot be known. A day of decision, a day of recompence, a day that will put a final period to all the days of time.
The world will be judged in righteousness for God is not unrighteous, who taketh vengeance far be it from him that he should do iniquity. His knowledge of all men's characters and actions is infallibly true, and therefore his sentence upon them incontestably just. And, as there will be no appeal from it, so there will be no exception against it.
God will judge the world by that man whom he hath ordained, who can be no other than the Lord Jesus, to whom all judgment is committed. By him God made the world, by him he redeemed it, by him he governs it, and by him he will judge it.
God's raising Christ from the dead is the great proof of his being appointed and ordained the Judge of quick and dead. His doing him that honour evidenced his designing him this honour. His raising him from the dead was the beginning of his exaltation, his judging the world will be the perfection of it and he that begins will make an end. God hath given assurance unto all men, sufficient ground for their faith to build upon, both that there is a judgment to come and that Christ will be their judge the matter is not left doubtful, but is of unquestionable certainty. Let all his enemies be assured of it, and tremble before him let all his friends be assured of it, and triumph in him.
The consideration of the judgment to come, and of the great hand Christ will have in that judgment, should engage us all to repent of our sins and turn from them to God. This is the only way to make the Judge our friend in that day, which will be a terrible day to all who live and die impenitent but true penitents will then lift up their heads with joy, knowing that their redemption draws nigh.