أَلَمْ يَأْنِ لِلَّذِينَ آمَنُوا أَن تَخْشَعَ قُلُوبُهُمْ لِذِكْرِ اللَّـهِ وَمَا نَزَلَ مِنَ الْحَقِّ (Has the time not yet come for those who believe that their hearts should be humble for the remembrance of Allah and for the truth that has descended [ through revelation ]?...57:16) The phrase khushu'-ul-qalb means 'for the heart to soften; to be receptive to advice; to be obedient; to submit'. [ Ibn Kathir ] In the Qur'anic context, it implies 'a complete willingness to yield to the Divine injunctions, or the Divine imperatives and prohibitions without allowing any laxity in following them. [ Ruh-ul-Ma’ ani ]
This verse admonishes the believers. Sayyidna ` Abdullah Ibn ` Abbas ؓ reports that some of the believers were found to be lacking in firmness, and somewhat laziness was felt in their practices. So this verse was revealed. Imam A'mash (رح) said: After settling down in Madinah, the blessed Companions ؓ experienced socio-economic comfort and prosperity; as a result some of them relaxed in exerting their efforts to do good works as they used to do previously. Thus this verse was revealed. [ Ruh-ul-Ma’ ani ]
Sayyidna ` Abdullah Ibn 'Abbas ؓ is narration also carries the additional information that this admonitory verse was revealed thirteen years after the revelation had started. [ Recorded by Ibn Abi Hatim (رح) ] Sayyidna ` Abdullah Ibn Masud ؓ narrates that this admonitory verse was revealed four years after they had embraced Islam to administer this mild reproof. Allah knows best!
In any case, the Muslims are cautioned in this verse that they should prepare themselves to turn to Allah totally and completely, and act upon the teachings of their religion. All actions revolve around khushu`-ul-qalb Sayyidna Shaddad Ibn Aws ؓ narrates that the Messenger of Allah ﷺ said: "The first thing that will be taken away from the people will be their humility or humbleness [ khushu']." [ Ibn Kathir ]