Romans 4:7-8 is in reference to past sin that is confessed and forsaken. Nowhere does the Bible teach that future sin is forgiven for a believer. 1 John 1:9 says “if we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from all unrighteousness.”
2 Peter 1:9 says, “But he that lacketh these things is blind, and cannot see afar off, and hath forgotten that he was purged from his old sins.”
Believers who justify sin can be in the kingdom and then they can later be cast out due to their justifying sin or iniquity. How so?
Matthew 13:41-42 says “The Son of man [i.e. Jesus] shall send forth his angels, and they shall gather out of his kingdom all things that offend, and them which do iniquity [i.e. sin]; And shall cast them into a furnace of fire [i.e. the Lake of Fire]: there shall be wailing and gnashing of teeth.”
In other words, a believer can be in the kingdom of Christ, but they can be cast out later at the judgment if they do iniquity. Christ's angels will gather those in Christ's kingdm who do iniquity and they will be cast into the Lake of Fire.