From John MacArthur's commentary on Ephesians, p. 209, emphasis MacArthur:
(for the fruit of the light consists in all goodness and righteousness and truth), trying to learn what is pleasing the Lord. (5:9-10)
In what appears to be a parenthetical statement, the manifest characteristics of the children of light are given in what Paul here calls the fruit of the light. . . The three supreme characteristics, or fruit, of our walk as children of light are all goodness and righteousness and truth.
These are the tests of true faith, of a true saving relationship to the Lord Jesus Christ. A "decision" for Christ, church membership, faithful attendance at worship services, being baptized, financial support of the Lord's work, and many other such things are often used as evidence of salvation. The faithful Christian should do all of those things, but they are behaviors that are easily done in the flesh and are therefore unreliable in themselves as evidence. On the other hand, the three characteristics Paul mentions here are spiritual works that cannot be achieved in the flesh. The all reflects the perfection of the divine standard.