The Common Understanding
I endeavor in my studies to seek out the truths of scripture by studying verses, passages, books, etc. in context. In the case of 1 John 1:9, I have not been comfortable with the common interpretation and application of this popular verse.
If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.
Pretty straightforward, one would think, but there are a few concerns I have with its blanket application. I'll quickly address the most obvious: this can only refer to true Christians. While that may seem obvious, it is important to note that almost the entire Bible is written to believers. This is especially important to note when dealing with the epistles. We start, then, with noting this is given to sanctified and secure believers. That "security" will be important as we look at the greater passage.
Beyond the seemingly obvious application to believers, we need to look at the surrounding context and John's audience. As with a few other commonly-quoted verses, the accepted understanding makes less sense when we look at the whole chapter and even the whole book in which we find it. We saw this in our study on 2 Cor 5:8 (The Most Misquoted Verse in Scripture).
The accepted understanding is that when a believer sins, he must confess that sin (to God) and then he will be cleansed from the unrighteousness brought on by that sin. As we go forward, let us remember this is supposedly in regard to sanctified and secure believers.
Cleanse and Cleansing
Let me lay some groundwork by pulling out the word "cleanse." We need to note here verse 7 which also includes this idea.
But if we walk in light, even as he is in light, then we have fellowship with him, and the blood of Jesus Christ his Son cleanses us from all sin.
Both verses employ the Greek word "katharízō." It is in the aorist tense, which, for our sake, means it is not locked into past, present, or future.
Something like, "If we confess our sins, he is faithful and just to forgive us our sins, and we are continually being cleaned from all unrighteousness."
A very good consideration to explain this verse is along this line: when we "sin," we must agree with God that what we have done is sin. This is born out in the following verse, "if we say we have not sinned, we make him a liar, and his word is not in us." This is slightly problematic for me as it is possible for a Christian to sin and not fully realize it. But, again, it's a reasonable argument.
Walk in the Light
That said, I think if we widen the lens, we start to see that the passage is addressing a past action. That is, being in the light (verse 7) leads to a continual action of cleansing.
But if we walk in light, even as he is in light, then we have fellowship with him, and the blood of Jesus Christ his Son [continually] cleanses us from all sin.
This verse is the condition. So what does it mean to "walk in the light?" Conversely, what does it mean to "walk in darkness" (verse 6)? John and the Lord give us the answer, I believe, in John's gospel.
In him was life, and the life was the light of men. And the light shines in the darkness, but the darkness comprehended it not. -John 1:4-5
Then Jesus spoke to them again, saying, I am the light of the world. He who follows me shall not walk in darkness, but will have the light of life. -John 8:12
I am come a light into the world, so that whosoever believes on me will not remain in darkness. -John 12:49
From these verses, we can see that one is in "darkness" until he truly has faith. We note here that john's gospel is written to the world (those in darkness) while his epistle is written to believing Jews in the Acts age. We will get to chapter 2 of 1 John soon, but let me dip my toe in that water while we're looking at "darkness."
Again, a new commandment I write to you, which thing is true in Him and in you, because the darkness is passing away, and the true light is already shining. -1 John 2:8
This "darkness," which is "passing away" is the darkness which was upon Israel. Matthew in his gospel of the kingdom to Israel alone quotes Isaiah in regard to the coming of the Messiah to his "people" (Israel):
The people who sat in darkness have seen a great light, And upon those who sat in the region and shadow of death, Light has dawned.” -Matt 4:16 (Is 9:2)
So, this is still the kingdom period when the Light of the Messianic kingdom in Israel was still anticipated. The Light of that kingdom was shining in the Acts Age as the kingdom was being offered (from Pentecost to the end of the Acts).
Next time we will take the idea of "darkness" being unbelief and "light" being faith and the acceptance of the offer of the finished work of the Savior and apply it 1 John chapter 1 as a whole.