وَإِذَا لَقُوا الَّذِينَ آمَنُوا قَالُوا آمَنَّا وَإِذَا خَلَا بَعْضُهُمْ إِلَىٰ بَعْضٍ قَالُوا أَتُحَدِّثُونَهُم بِمَا فَتَحَ اللَّهُ عَلَيْكُمْ لِيُحَاجُّوكُم بِهِ عِندَ رَبِّكُمْ ۚ أَفَلَا تَعْقِلُونَ
Wa izaa laqul lazeena aamanoo qaalooo aamannaa wa izaa khalaa ba'duhum ilaa ba'din qaalooo atuhaddisoo nahum bimaa fatahal laahu 'alaykum liyuhaajjookum bihee 'inda rabbikum afalaa ta'qiloon
When they meet those who believe, they say, "We believe." But when they are alone with one another, they say, "Do you tell them what God has disclosed to you, so they can use it against you before your Lord? Have you no sense?"
The verse works by putting two rooms side by side, one where a claim is made and one where the reason for regretting it is discussed. What the private speech worries about is the usefulness of the disclosure to the other side in an argument, rather than the belief itself. The complaint that closes it is directed by the speakers at each other, which is the only place in the passage where that happens.