The Believers are tested so that it may be known Who is Sincere and Who is Lying

أَحَسِبَ النَّاسُ أَن يُتْرَكُواْ أَن يَقُولُواْ ءَامَنَّا وَهُمْ لاَ يُفْتَنُونَ

وَلَقَدْ فَتَنَّا الَّذِينَ مِن قَبْلِهِمْ فَلَيَعْلَمَنَّ اللَّهُ الَّذِينَ صَدَقُواْ وَلَيَعْلَمَنَّ الْكَـذِبِينَ

Does mankind think that by (just) saying "We believe," they will be left alone and that they will not be tested? And We indeed, did test those who were before them, so that Allah will know, those who are true and He will know those who are liars. - Sura Al-Ankabut 29:2-3