Christians are not old fashioned. They are sinners (corrupt programs) who choose to be born again by accepting Jesus Christ as their Savior. That means re- born not of the flesh but of the Spirit and thus Saved. They will rein in the New Jerusalem with Christ.
The best way to imagine this is to consider a person as a computer program that has been corrupted by a virus. Only those 'persons' codes in the program that accept the 'patch' (fix for the corruption - Jesus Christ) can be and will be saved by the Creator of the program.
Those that accept the patch 'fix for corruption' will be brought into the new program - the new creation because the corruption that was in them has been covered by 'the Blood of the Lamb' the patch, the God given upgrade which gives them access to the Kingdom of Heaven. There is no way 'access' to the Kingdom of the Father other than through Jesus Christ. John 14:6.
This is not old fashioned thinking. This requires a new mind! A new social imagination. If you don't have the desire to make the choice for this, you will not have it nor be saved. This is because God the Creator of Heaven and Earth gave us free will. That means we have the choice to be saved. In this way, by our own choice, we will be able to access the Kingdom of Heaven.
"Praise the Lord, O my soul. I will praise the Lord all my life: I will sing praise to my God as long as I live. Do not put your trust in princes, in mortal men who cannot save..." Psalm 146