I John 1:9 says that "If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness." but would that apply to someone who's a baptized Christian but has been away from God for several months, committed fornication and has been very close to atheism? Mostly the atheism part. The Bible says a Christian is the temple of the Holy Spirit, but what if this person's been away for so long? What happens to the Holy Spirit? Does He go away and comes back?
A person who once was a Christian and now seems just as worldly as anybody else?
If the person wants to come back, how can he pray or worship God as a Christian and not as a sinner?
God doesn't look at life and time the way we do. We are the ones who get impatient and give up on people. To someone who lives an eternity, your entire lifetime isn't that long.
"Seek the LORD while He may be found, call upon Him while He is near. Let the wicked forsake his way, and the unrighteous man his thoughts; let him return to the LORD, and He will have mercy on him; and to our God, for He will abundantly pardon. "For My thoughts are not your thoughts, nor are your ways My ways," says the LORD. "For as the heavens are higher than the earth, so are My ways higher than your ways, and My thoughts than your thoughts"" (Isaiah 55:6-9).
God is seeking to save people, just like you, who want to leave the bondage of sin. The difference between a saint and a sinner is that the sinner doesn't care about leaving his sins behind. A saint won't stay in sin. "For a righteous man may fall seven times and rise again, but the wicked shall fall by calamity" (Proverbs 24:18).
God didn't leave, you left God (Isaiah 59:1-2). God is waiting for you to come home -- that is one of the messages of the prodigal son (Luke 15:11-32) -- so why are you dragging your feet? And if you don't feel worthy to approach God, that is what your brethern are here to help you with. "Confess your trespasses to one another, and pray for one another, that you may be healed. The effective, fervent prayer of a righteous man avails much" (James 5:16).