(Al-Fath'h) 48 : 4
هُوَ الَّذِي أَنْزَلَ السَّكِينَةَ فِي قُلُوبِ الْمُؤْمِنِينَ لِيَزْدَادُوا إِيمَانًا مَعَ إِيمَانِهِمْ ۗ وَلِلَّهِ جُنُودُ السَّمَاوَاتِ وَالْأَرْضِ ۚ وَكَانَ اللَّهُ عَلِيمًا حَكِيمًا
He is the one who sent down tranquility in the hearts of the believers, for increasing their belief with their belief; and to Allah belongs the armies of the heavens and the earth, while Allah is the all knower, the Wise.
Only with Adhikr-the Permission of Allah mentioned in verse 10: 100-one can become believer and increase his belief. Only with It one will get tranquility to the heart as explained in verse 13: 28. The believers utilize Adhikr as the Ticket to the Paradise and Shield and safeguard against all calamities, disasters, illness and hell fire. See explanation 9: 40; and 47: 17.