(NIV) I am writing to you, dear children, because your sins have been forgiven on account of his name.
(NET) I am writing to you, little children, that your sins have been forgiven because of his name.
The next three verses use the phrase “I am writing to you, dear children” joined with the conjunction “because” or “that”, meaning that John is either telling us why he is writing these things, or he is writing this series of admonishments to remind his readers of something very important.
ὅτι (hoti): this word demonstrates either the “reason” or the “content” of a statement and is similar to our English words “because” or “that”, depending on the context, which means we have to consider the context of John’s entire epistle to determine whether “ὅτι”means “because”, which expands on why John wrote his first epistle, or “that”, which gives us important spiritual exhortations for the Church body.
Reasons for “BECAUSE”: John writes a series of “reasons for writing” in 2:1, 12-14, 26, that could be understood as building on, and clarifying his main prologue points. This would mean that John is writing this letter because they are forgiven (v12), because they know God (v. 13, 14), have overcome Satan (v. 13, 14), are strong (v.14), and God’s word abides in them (v. 14). This might suggest that John would not be writing these things to the unregenerate, perhaps because he would see no reason or benefit in doing so.
Reasons for “THAT”: John introduces his reasons for writing in his prologues and everything unfolds from there, making it highly unlikely that John is introducing a new reason for writing his epistle and more likely John is writing this letter to remind them that they are forgiven (v12), that they know God (v. 13, 14), that they have overcome Satan (v. 13, 14), that they are strong (v.14), and that God’s word abides in them (v. 14). This might suggest that John’s goal is to remind the Church of Christ’s redemptive work in them.
My Conclusions: I find it less likely that John is informing his readers that he is writing these things because they have been redeemed and more likely that John is writing this series of admonitions to remind them of Christ’s redemptive work being accomplished in them. The multiplicity of tests and verifications that John provides in his first epistle suggest that his primary focus is on extolling and verifying Christ’s powerful redemptive work in the Church.
Therefore, the rendering that I find most accurate in this context comes from the New English Translation:
1st John 2:12 (NET) I am writing to you, little children, that your sins have been forgiven because of his name.
“Little Children”: this is the affectionate address that John uses to the collective recipients of his letter (see 2:1), and should not be understood as being limited to a specific group within the Church.
This exhortation is the foundation of the “eternal life” (see 1:2) that John is writing about in this letter and returns to the all-important doctrinal instructions about the atoning sacrifice of Christ in 2:1-2. They have confessed their sins (1:9) and on account of faith in him and his name (3:23; 5:1, 5, 13) forgiveness was purchased through the covenant of his blood (1:7). In this knowledge, they may stand firm. Because they are forgiven, they may also have fellowship with God and true knowledge of him (13c).