Is faith in the heart alone sufficient for a Muslim to be considered a Muslim, without prayer, fasting, or zakat?

Belief in the heart alone is not sufficient; rather, it is necessary to believe in Allah and His Messenger and in all that is revealed in the Quran and the Sunnah, to utter the two testimonies of faith (Shahadatayn), and to establish prayer. Abandoning prayer is considered a major disbelief by many scholars, based on the Prophet's (peace be upon him) saying: "Between a person and shirk (polytheism) and disbelief is the abandonment of prayer," and "The covenant between us and them is prayer; whoever abandons it has disbelieved." As for abandoning the rest of the pillars, such as fasting, zakat, and Hajj, while acknowledging their obligation, it is a grave sin and a deficiency in faith, but not major disbelief.