Some words and statements are so precious to Allah subhanahu wa ta’ala that a person utters those words and Allah rewards them with Jannah.
فَأَثَابَهُمُ اللَّهُ بِمَا قَالُواْ جَنَّـتٍ تَجْرِى مِن تَحْتِهَا الاٌّنْهَـرُ خَـلِدِينَ فِيهَا وَذَلِكَ جَزَآءُ الْمُحْسِنِينَ
So because of what they said, Allah awarded them with Gardens under which rivers flow, they will abide therein forever. Such is the reward of good-doers.
When the Christians accepted Islam upon hearing the recitation they said what is there not to believe. They then made a du’a to Allah subhanahu wa ta’ala to accept them among the righteous people. Allah subhanahu wa ta’ala loved their declaration of emaan and du’a so much that He rewarded them with Jannah.
This ayah teaches us the importance of the words that we say. We should keep a check on every word that we utter. When we are shouting and yelling we should listen to our own words what are we saying. Our words can ruin relationships or mend hearts.
This ayah also teaches the reward of ehsaan. Ehsaan includes speech. They were called muhsineen because of what they said. Ehsaan is not limited to being generous or giving money. It is about the excellence of our conduct. Whatever we do we should do it consciously reminding ourselves that Allah is watching.
In ayah 86, Allah subhanahu wa ta’ala mentions another group of people.
وَالَّذِينَ كَفَرُواْ وَكَذَّبُواْ بِـَايَـتِنَآ أُوْلَـئِكَ أَصْحَـبُ الْجَحِيمِ
But those who disbelieved and belied Our Ayat, they shall be the dwellers of Hell.
These people deny the favors of Allah, reject His commands and utter lies. Such are the people who will be the dwellers of Hell. For example, people who say there’s no need to offer five daily prayers or those who deny the Day of Judgment.
Kufr can take a person to Hellfire. We should watch our speech and not utter lies about something we don’t know.