Unfruitfulness is part of the nature of the unbeliever, especially those who think they are Christians, but are only deceived. These are the ones who wreak havoc in the visible church with their unfruitful works of darkness.
6 Let no one deceive you with empty words, for because of these things the wrath of God comes upon the sons of disobedience. 7 Therefore do not be partakers with them; 8 for you were formerly darkness, but now you are Light in the Lord; walk as children of Light 9 (for the fruit of the Light consists in all goodness and righteousness and truth), 10 trying to learn what is pleasing to the Lord. 11 Do not participate in the unfruitful deeds of darkness, but instead even expose them; 12 for it is disgraceful even to speak of the things which are done by them in secret. Ephesians 5:6-12 (NASB)
In the passage above is the the word “fruit.” It is a translation of the Greek word καρπὸς (karpos). This word appears some sixty-six times in the New Testament. In most cases, it is used in reference to the the fruit of plants as in Matthew 21:19 or the produce of the earth (James 5:7, 18). However, its extended meaning in the New Testament is one in which we should pay special attention as well as to its opposite, ἀκάρποις (akarpois), which is translated in this passage as “unfruitful.” In this form, ἀκάρποις is the dative, plural of ἄκαρπος (akarpos). The dative plural simply means that this part of the phrase is an indirect object of the action of the sentence. In any case, this may be more significant than it first appears.
6 Μηδεὶς ὑμᾶς ἀπατάτω κενοῖς λόγοις· διὰ ταῦτα γὰρ ἔρχεται ἡ ὀργὴ τοῦ θεοῦ ἐπὶ τοὺς υἱοὺς τῆς ἀπειθείας. 7 μὴ οὖν γίνεσθε συμμέτοχοι αὐτῶν· 8 ἦτε γάρ ποτε σκότος, νῦν δὲ φῶς ἐν κυρίῳ· ὡς τέκνα φωτὸς περιπατεῖτε 9 — ὁ γὰρ καρπὸς τοῦ φωτὸς ἐν πάσῃ ἀγαθωσύνῃ καὶ δικαιοσύνῃ καὶ ἀληθείᾳ — 10 δοκιμάζοντες τί ἐστιν εὐάρεστον τῷ κυρίῳ, 11 καὶ μὴ συγκοινωνεῖτε τοῖς ἔργοις τοῖς ἀκάρποις τοῦ σκότους, μᾶλλον δὲ καὶ ἐλέγχετε. 12 τὰ γὰρ κρυφῇ γινόμενα ὑπʼ αὐτῶν αἰσχρόν ἐστιν καὶ λέγειν, Ephesians 5:6-12 (NA28)