(Part No. 1; Page No. 735)
Abandoning Salah is an act of Al-Kufr-ul-Akbar (major form of disbelief) even if one does not deny its obligation. This is the most proper viewpoint of scholars. The Prophet (peace be upon him) said:
That which differentiates us from the Kafirs (disbelievers) and hypocrites is our performance of Salah. He who abandons it, becomes a Kafir (disbeliever).
There are other Hadiths of the same meaning.
Committing Zina, dealing in Riba and drinking Khamr are from the major sins that do not render a Muslim a Kafir as long as he does not regard them as lawful. But committing these sins puts a person in a great danger. If someone dies as persisting in committing major sins, they are under Allah's Will. If He wills to forgive them, He will forgive them. Allah may also punish them according to their sins and then admit them to Jannah (Paradise). Allah (Exalted be He) says:
Verily, Allâh forgives not that partners should be set up with Him (in worship), but He forgives except that (anything else) to whom He wills
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.