If ye forsake the LORD, and serve strange gods, then he will turn and do you hurt, and consume you, after that he hath done you good.

Wesley's Notes

24:20 Will turn - That is, he will alter his course and the manner of his dealing with you, and will be as severe as ever he was kind and gracious. He will repent of his former kindnesses, and his goodness abused will be turned into fury.

Matthew Henry's Concise Commentary

24:15-28 It is essential that the service of God's people be performed with a willing mind. For LOVE is the only genuine principle whence all acceptable service of God can spring. The Father seeks only such to worship him, as worship him in spirit and in truth. The carnal mind of man is enmity against God, therefore, is not capable of such spiritual worship. Hence the necessity of being born again. But numbers rest in mere forms, as tasks imposed upon them. Joshua puts them to their choice; but not as if it were indifferent whether they served God or not. Choose you whom ye will serve, now the matter is laid plainly before you. He resolves to do this, whatever others did. Those that are bound for heaven, must be willing to swim against the stream. They must not do as the most do, but as the best do. And no one can behave himself as he ought in any station, who does not deeply consider his religious duties in family relations. The Israelites agree with Joshua, being influenced by the example of a man who had been so great a blessing to them; We also will serve the Lord. See how much good great men do, by their influence, if zealous in religion. Joshua brings them to express full purpose of heart to cleave to the Lord. They must come off from all confidence in their own sufficiency, else their purposes would be in vain. The service of God being made their deliberate choice, Joshua binds them to it by a solemn covenant. He set up a monument of it. In this affecting manner Joshua took his last leave of them; if they perished, their blood would be upon their own heads. Though the house of God, the Lord's table, and even the walls and trees before which we have uttered our solemn purposes of serving him, would bear witness against us if we deny him, yet we may trust in him, that he will put his fear into our hearts, that we shall not depart from him. God alone can give grace, yet he blesses our endeavours to engage men to his service.