The Bible is very clear that I, as a Born again believer have been passed from eternal death unto eternal life right now because Jesus took all my sins, including future sins, to the Cross and dealt with them there once for all, never to bring them up again to my presence for ever. The above being the truth, as also given in Rom. 8:1, why is 1 John 1:9 applied to born again believers as well?
There are two facets to our relationship with God. One is our salvation which you correctly describe as having been made certain forever at the cross. It pertains to eternity. The other facet is our relationship with God in the here and now, on Earth. Being born again makes you a child of God and nothing can ever change that. But since we still have a sin nature, we still sin, and when we do, it can strain our earthly relationship with God, and may mean we’ll miss out on blessings we might otherwise receive.
John was writing to born again believers who had already been saved. He told them that if they claimed to no longer sin they weren’t being truthful, but if they confessed their sins God is just and faithful and would forgive them and purify them from all unrighteousness (1 John 1:8-9). That meant they could prevent their earthly relationship with God from being strained by confessing whenever they sinned. So it is with us. Some call 1 John 1:9 the Christian’s bar of soap for this reason.