Verse 18. In everything give thanks. See Barnes "Eph 5:20"; See Barnes "Php 4:6".

We can always find something to be thankful for, and there may be reasons why we ought to be thankful for even those dispensations which appear dark and frowning. Chrysostom, once the archbishop of Constantinople, and then driven into exile, persecuted, and despised, died far away from all the splendours of the capital, and all the comforts and honours which he had enjoyed, uttering his favourite motto doxa tw yew pantwn eneken glory to God for all things. Bibliotheca Sacra, i.700. So we may praise God for everything that happens to us under his government. A man owes a debt of obligation to him for anything which will recall him from his wanderings, and which will prepare him for heaven. Are there any dealings of God towards men which do not contemplate such an end? Is a man ever made to drink the cup of affliction when no drop of mercy is intermingled? Is he ever visited with calamity which does not in some way contemplate his own temporal or eternal good? Could we see all, we should see that we are never placed in circumstances in which there is not much for which we should thank God. And when, in his dealings, a cloud seems to cover his face, let us remember the good things without number which we have received, and especially remember that we are in the world of redeeming love, and we shall find enough for which to be thankful.

For this is the will of God. That is, that you should be grateful. This is what God is pleased to require you to perform in the name of the Lord Jesus. In the gift of that Saviour he has laid the foundation for that claim, and he requires that you should not be unmindful of the obligation. See Barnes "Heb 13:15".