What is the ruling on one who knew that delaying the 'Isha prayer until after midnight was forbidden, then learned that it was permissible, and so sometimes became lazy and prayed it after midnight? And is her decision to delay the prayer until after midnight considered disbelief (kufr)? And what is the ruling on her ghusl (ritual bath) after this decision?

The time after midnight is for the 'Isha prayer, and it is a time of necessity according to some jurists. Whoever intentionally delays it until then without a valid excuse commits a sin, but their prayer is considered performance (adaa') and not a make-up (qadaa'). According to the majority of scholars, there is no sin upon one who delays the 'Isha prayer until this time. The opinion that its time ends at midnight is weak. The action of the questioner does not entail disbelief.