ERSE 2 is key in understanding the point John is making in chapter. Notice the timing element in John's statement. We are already the children of God, now; presently! But there is also something that we aren't yet. We aren't yet perfectly holy like Jesus is. We will be someday. When we see Jesus at His coming (in the rapture), we will be changed (1st Corinthians 15:51 & 52)! We will become completely godly; no longer susceptible to temptations and failures. But for right now, we still lack some things.
Verse 3 is almost as important as the previous verse in this context. Everyone who is looking sincerely for Christ to return will be in the process of self-purification (1st John 3:2). We wouldn't need to purify ourselves if we were already pure and somehow incapable of transgressions. The point of both of these verses is to admit that we are still works-in-progress. This is the foundation for the great calls to sanctification that fill the remainder of this chapter. Without 1st John 3:2 & 3, this chapter would be a bit harder to swallow. But let's not yank the teeth out of the shark here.
The salient point here is that saints cannot continually revel in sin and get away with it. If sin dominates a man professing to be a saint, one of two things should be expected. 1. The saint is actually an ain't or 2. retribution and/or revival is just around the corner. If we are God's children, then God will deal with us like family. If we are imposters or pretenders, then no wonder we can't quite fit in. Saints just can't hang happily and continually with the devil (vs. 6). Like it or not, on the authority of God's Word we must admit that salvation either has a real impact on behavior, or it isn't salvation at all (vs. 3:7, 8 & 10).