In 1 John 1:5–10, John is speaking to believers, those who “have fellowship with one another” and God (1 John 1:7; cf. 1:3). In 1 John 1:9 it is these believers who “confess” (homologeō) their “sins,” obviously to God. Broken apart, the Greek verb homologeō literally means “to say the same thing,” and its use in context indicates who is agreeing and what the matter of agreement happens to be. The matter of agreement here concerns sins, and the two that should agree about this matter are God and believers. To “confess our sins” is to agree with God about our sins, which is more or less the same as repentance.
Many understand 1 John 1:9 to speak of unbelievers who confess their sins in order to be saved and have fellowship with God. According to this understanding, if the confession of sins is necessary to having fellowship with God, then this fellowship can apparently be lost by engaging in acts of sin after salvation. Stated another way, walking the darkness puts believers out of fellowship with God (1 John 1:6).
From this understanding, the application for the believer is that he can lose his fellowship through sin after the point of salvation. If fellowship with God is equivalent to salvation (i.e., possessing eternal life; cf. 1 John 1:1–4), then those who hold to eternal security (once saved, always saved) must redefine fellowship in order to avoid having a believer lose his salvation through sin. The solution typically involves creating a category of believers who apparently have no fellowship with God and walk in darkness (cf. 1 John 1:6). Fellowship effectively becomes something less than salvation and something only enjoyed by believers who have confessed any sins committed after salvation. Practically, this understanding of confession and fellowship can tend towards methods that pressure Christians for crisis moments of confession in order for them to regain their fellowship with God.
As stated above, however, a correct understanding of 1 John 1:5–10 identifies believers as those who confess their sins in 1 John 1:9. Since fellowship involves sharing God’s eternal life and is thus equivalent to salvation (cf. 1 John 1:1–4), those who hold to eternal security naturally hold that this fellowship cannot be lost. This being the case, the sins to confess in 1 John 1:9 must be occasional and something less than a habitual walking in darkness that is only true of unbelievers who have no fellowship with God (cf. 1 John 1:6).
When we as believers occasionally sin, we hinder our relationship with God but do not lose our fellowship with Him. When we think of these sins as God does, “we confess our sins” to God so He can “forgive us our sins” and “cleanse us from all unrighteousness” (1 John 1:9), something distinct from the once-for-all forgiveness and cleansing given at the initial point of salvation (cf. 1 Cor 6:11).