It's enough to be a Muslim to say shahadatain, not for being a faithful.
Question:
Is there any difference between Mu’min & Muslim in holy Qur’an?
Answer:
Allah (SWT) says: “The bedouins say, "We have believed." Say, "You have not [yet] believed; but say [instead], 'We have submitted,' for faith has not yet entered your hearts. “[1]
According to the verse above the difference between Islam and faith is that the Islam has the appearance and the legal form. And everyone who says Shahadatain [2] is considered as a Muslim and Islamic laws include him. But the faith is a real and inner thing and his place is human’s heart not tongue or appearance.
Note:
[1] Surah al-HUJURAT, verse 14 : “قالَتِ الْأَعْرابُ آمَنَّا قُلْ لَمْ تُؤْمِنُوا وَ لكِنْ قُولُوا أَسْلَمْنا وَ لَمَّا يَدْخُلِ الْإيمانُ في قُلُوبِكُمْ”
[2] “I bear witness that there is no god but Allah” & “I bear witness that Muhammad is the Messenger of Allah”