We may be interpreted as inclusive, referring to John and the congregation he is addressing, or exclusive, referring to the eyewitnesses of the word (compare comments on 1.1-4). The former is preferable.

Whoever knows God: for “to know God” (also occurring in verses 7-8) see comments on “we know him” in 2.3. The meaning of the expression comes very close to that of “to be of God”; both refer to an intimate personal relationship.

By this: the preposition By, in the Greek literally “out-of,” indicates a source. The demonstrative pronoun this points back. The phrase marks the facts mentioned in the preceding sentence as being the source from which knowledge about the Spirit can be derived; from a person’s listening or not listening to the message, we can learn whether he is inspired by the spirit of truth or the spirit of error. Compare also verses 2-3, where a man’s confessing or not confessing Jesus is mentioned as the means to know whether or not he has God’s Spirit. Some renderings used are “that is how” (New English Bible), “in this way” (Goodspeed), “because of that” (Good News Translation for the same phrase in John 6.66).

We know or, in this context, “we can tell the difference between” (Good News Translation).

The spirit of truth and the spirit of error: the construction with of allows various interpretations. The spirit may be viewed as inspiring (or leading to) truth or error, that is, as causing people to say what is true, or to lie/deceive. Or one may take the spirit as having the essential quality of truth or error, that is, as being true or not true.

The meaning spirit has here belongs to category (4) as mentioned in the comment on verse 1. For truth see comments on 1.8. The Holy Spirit is sometimes called the Spirit of Truth; for example, in John 14.17.

Error: the corresponding Greek term is related to the verb rendered “to deceive” in 1.8. The meaning the word has here is determined by its being the direct opposite of truth. Error and untruth characterize the sphere of the Devil, who is called “a liar and the father of lies” in John 8.44.

Quoted with permission from Haas, C., de Jonge, M. and Swellengrebel, J.L. A Handbook on The First Letter of John. (UBS Handbook Series). New York: UBS, 1972. For this and other handbooks for translators see here .