Taqwa is the faithful’s fortress.
Taqwa (تقوى) is defined in Islam as the avoidance of sin. It is roughly translated into the English concept of piety and God-fearing. The Holy Quran says in Surat Al-Baqarah
And take provisions, but indeed, the best provision is fear of Allah
Taqwa is the subject of many sermons by Ali bin Abi Talib. In sermon 156 of Nahjul Balaghah he says:
Know O’ creatures of Allah! that piety is a strong house of protection while impiety is a weak house which does not protect its people, and does not give security to him who takes refuge therein.