truly in him
The word order used here as “truly in him” is better than “in him the love of God has truly…” because it more closely reflects the word order of the original text (ἀληθῶς ἐν τούτῳ; alēthōs en toutō). “Truly” is fronted here for more prominent emphasis, as it is the person who keeps the word of God who can have full assurance of eternal life since truly the love of God has been perfected in this person.

blinded
While some translations say “has blinded,” this would give the sense of a perfect tense verb. The Greek has a simple aorist tense, which is translated as “blinded.” While the perfect tense places a greater emphasis upon the resultant state of a past event or action, the aorist places the emphasis upon the action itself. Therefore, in this context, readers can better see that John is highlighting the fact of the darkness blinding the eyes of this person, and not so much the fact that this individual is currently blind. By clarifying the emphasis even slightly, readers can better sense the blinding power of the darkness itself.

have known
While some translations simply have “know” here, the translation “have known” accurately carries over the sense of the perfect tense verb ἐγνώκατε (egnōkate) here. This is chosen over something like “have come to know” (e.g., vv. 3, 4) because the verb has a consummative sense in this context. The “fathers” have relatively more experience in Christ than the others as they have known Him over a longer period of time. In the case of “the children,” the perfect verb is translated consistently and emphasizes the state of knowledge of the Father they have because they are born of Him.

have known
Refer to the discussion on verse 13 above.

abides
“Abides” provides a more consistent translation of the Greek term μένω (menō; cf. 2:6, 10, 14, 24, 27, 28, 3:6, 9, 14, 15, 17, 24; 4:12, 13, 15, 16). While some translations render this as “lives,” in contrast to “passing away,” John’s term is more nuanced and focuses on the reality of remaining, enduring, or staying. Thus, unlike the world, which is temporary and currently passing away, the person who does the will of God is a part of the new creation, which will remain when all else in this fallen world has passed away (cf. Heb 12:27–29).

were
Some translations render this in the pluperfect tense in English (i.e., “had been”), but the Greek verb is in the imperfect tense (ἦσαν; ēsan), which usually communicates a continual or ongoing sense. Thus, the translation as “were” better shows readers the contrast that John makes in relation to how these apostates “were not of us.” In saying this, John looks back to describe the ongoing state of how these people “were” when they were in physical fellowship with the saints. Their nature is understood in this case by whether they remained or abandoned the fellowship.

manifested
“Manifested” is a simpler, one-word translation of the Greek term φανερόω (phaneroō), which is translated as “manifested” in 1 John 1:2. Much like how the invisible Word of Life was manifested in time and space, the once-invisible heart attitudes of these apostates are shown in their actions of abandoning the church. The consistent translation of φανερόω (phaneroō) with “manifest” throughout 1 John helps the reader to recognize the same term everywhere it is used.

which you heard from the beginning
The word order of this sentence has been adjusted in order to better reflect that of the Greek. Additionally, the same general word order is repeated in the second sentence of this verse. John fronts the idea of what they heard from the beginning, and understanding that word order better prepares the reader to understand why John proceeds to write about letting that same message continue to abide in them, even after they first heard it.

about
“About” gives a straightforward translation of the Greek preposition περί (peri). The same preposition is used again in verse 27, where John mentions how God’s anointing teaches his readers “about” all things. The consistent translation helps readers to understand the way in which John draws attention to the content of something being taught.

whom
The translation here as for the relative pronoun ὃ (ho) as “whom” rather than “which” conveys that the “anointing” John speaks of here is a reference to the Holy Spirit, who is a person. While the Greek pronoun itself does not necessarily demonstrate personhood, the context of this verse and the understanding of “anointing” in the New Testament (which nearly always refers to the Spirit Himself) point to John’s intent here to reference the Spirit with his use of “anointing.” John’s teaching here about the indwelling and teaching of the “anointing” is also consistent with Jesus’ own teaching recorded in John’s Gospel about the indwelling and teaching of the Holy Spirit (cf. John 15:26; 16:13). Unlike apostates and false teachers, true believers have the Holy Spirit to teach them, and thus do not need to rely on any wisdom or teaching that originates from men. Believers should rely on the teaching of the Spirit and abide in Him rather than following after the divergent and deceptive teaching of apostates.

He
See previous note. Translating this third person singular verb with “He” rather than “it” reflects the understanding that the subject in view here is “His anointing,” which, as argued above, refers to the Holy Spirit.

abide
The Greek form of this verb “abide” (μένετε; menete) can be read either as a command or a statement of fact. In the translation here, the most likely option is given, with the alternative in a footnote. Here, rendering it as an imperative makes more sense of the context. One reason for this is grammatical, and the other contextual. The grammatical reason is that, as Culy points out, Paul uses the introductory phrase “But as” (ἀλλὰ ὡς; alla hōs; like John does here) to introduce “the grounds for an implicit command” in Ephesians 5:24 (Martin M. Culy, I, II, III John: A Handbook on the Greek Text, 61). While this is not determinative, it does give precedence for this manner of setting up a command. On top of that, a contextual reason for taking this as an imperative is that the same term appears again as a command in the next verse, which is tightly connected to this verse both by lexical similarities and John’s use of pronouns. In verse 28, John calls his readers to abide, but there he sets the command in the light of what is to come the future, unlike verse 27, where he (arguably) commands them to abide in light of their anointing in the past and His ongoing ministry to them in the present. Thus, from the context and flow of the passage, it appears that John is framing their perspective on life (past, present, future) in order to urge them, in light of all of that, to abide.

manifested
Refer to the discussion on verse 19 above.

does
John uses the term ποιέω (poieō; “do” or “make”) 13 times in the five chapters of this epistle (1:6, 10; 2:17, here; 3:4 (2x), 7, 8, 9, 10, 22; 5:2, 19), making it one of his key terms. It is translated consistently with “do” so that the reader can recognize it everywhere it appears (except in 1:10 and 5:10 where it is clearly “make”/”made”). Thus, John emphasizes throughout that a true believer who can have assurance of eternal life is a doer of the truth (here), the will of God (2:17), righteousness (2:29; 3:7, 10), what pleases God (3:21), and God’s commandments (5:2). Conversely, a true believer is not a doer of sin and lawlessness (3:4, 8, 9). This connects seamlessly with James’ theology that a true believer who has saving faith is a doer (using ποιέω [poieō]) of the word (James 1:22–25). In this verse, it is the person who does righteousness is easily identified as one who has been born of God.

has been born
The translation “has been born” captures the perfect tense of the Greek term (γεγέννηται; gegennētai). Since the perfect tense points to a present state and frames it in light of a past action, readers can see that John is careful to make sure his readers understand that a believer’s present and practical righteousness is derived from God in the new birth. If they are to live as children of God in the present, they must have been born of God in the past. Present righteousness is an outflow of the spiritual life received in the new birth. So by using this tense and keeping the past work of God in view, John helps readers look back to see that their own spiritual life and obedience is sourced wholly in their heavenly Father.