هُوَ ٱلَّذِىٓ أَنزَلَ ٱلسَّكِينَةَ فِى قُلُوبِ ٱلْمُؤْمِنِينَ لِيَزْدَادُوٓا۟ إِيمَـٰنًا مَّعَ إِيمَـٰنِهِمْ ۗ وَلِلَّهِ جُنُودُ ٱلسَّمَـٰوَٰتِ وَٱلْأَرْضِ ۚ وَكَانَ ٱللَّهُ عَلِيمًا حَكِيمًا
Huwal lazeee anzalas sakeenata fee quloobil mu'mineena liyazdaadooo eemaanamma'a eemaanihim; wa lillaahi junoodus samawaati wal ard; wa kaanal laahu 'Aleeman Hakeemaa
It was He who made His tranquillity descend into the hearts of the believers, to add faith to their faith––the forces of the heavens and earth belong to God; He is all knowing and all wise––
It is God who sent down tranquility into the hearts of the believers, so they would grow in faith upon their faith. To God belong the forces of the heavens and the earth, and God is all-knowing, all-wise.
Tranquility is named six times in the Qur'an and half of those fall in this surah. Wherever it is sent down, armies are mentioned alongside it: forces no one could see at 9:26 and 9:40, and here every host that belongs to God, which puts a calm placed inside a chest in the same category as troops. Whether faith can increase became a long argument among theologians, and this line is one of the places it was fought.