This short letter is primarily a precis of the First Letter of John. The two writings share a theological point of view and use similar phrasing. These simiilarities include:
There are also several differences between this letter and the First Letter of John. The most obvious is the letter's brevity, a trait shared with third letter. This, along with the veiled identity of the churches as "the Lady" and the the unwillingness to use "paper and ink", might suggest that the author is trying to be obscure in order to avoid persecutors. But in fact the letter refers openly to Jesus and to his status as God's son. Perhaps the simpler explanation, offered in notes to the New American Bible, is that the writer desired to limit the letter to a single page.
A more important difference is the emphasis given to the danger of deceivers. Not only does this topic occupy a greater percentage of this short letter, but the warning is more strident. "Some" people are walking the truth, while others are "running ahead" and, thereby, losing their way. Notwithstanding the admonition to love each other, the letter advises separating from those travelling teachers who are on the wrong side of this dispute.