John Chrysostom

That is, "let your requests be made known unto God, with thanksgiving" (Phil. iv. 6); for there is nothing so pleasing to God, as for a man to be thankful. But we shall be best able to give thanks unto God, by withdrawing our souls from the things before mentioned, and by thoroughly cleansing them by the means he has told us. What then? Are we to give thanks for everything that befalls us? Yes; be it even disease, be it even penury. …

Homily on Ephesians 19 ↗

Theophylact of Ohrid

"Always": not only in peace, but also in sorrow, and not only for what is good, but also for what is grievous, for what we know, and for what we do not know. For all things serve for our benefit, even though we do not realize it. And one must give thanks to the Father in the "name of our Lord Jesus Christ," that is, invoking the name of the Lord Jesus, and turning to Him as the mediator both of the benefit and of the thanksgiving itself.

Commentary on Ephesians ↗

Jerome

Paul now calls us to "give thanks always and in everything." This is to be understood in a double sense, both in adversity and in good times.… In this way the mind rejoices and bursts out in gratitude to God, not only for what we think good but for what troubles us and happens against our will.… It is obvious that generally we are called to give thanks to God for the sun that rises, for the day that goes by and for the night that brings rest … for the rains that come, for the earth that brings forth fruit and for the elements in …

Commentary on Ephesians 5:20 ↗

Thomas Aquinas

The third effect is thanksgiving because, when someone is influenced in these ways toward God, he recognizes that everything he has is from God. For the more a person is affected by his relation to God and knows him, the more does he see God as greater while he himself becomes smaller, indeed almost nothing, in comparison with God. "Now my eye seeth you. Therefore do I reprehend myself, and do penance in dust and ashes" (Job 42:5-6). …

Commentary on Ephesians ↗