There is no record that they were believers. Almost certainly they were not Christians. It appears they were faking it to be a part of the latest “fad” in Jerusalem. Everyone reaches a point where either they accept Christ, or, sadly, they reject Christ for the last time. The latter may have been the fate of Ananias and Sapphira posing as “fake” believers.
And there's no record they weren't believers.
That's just rationalization to make the event fit your theology.
Paul also gives instructions in Corinthians for the man who was in immorality with his mother-in-law.
God dealt with sin at the cross but that does not mean we can sin without consequence since then. That is demonic thinking. God will NOT bless a person, a people, or a nation that turns their back on Him and chooses to live in unrepentant sin.
1 Peter 4:17 For it is time for judgment to begin at the household of God; and if it begins with us, what will be the outcome for those who do not obey the gospel of God?