Do people think that they will be left alone because they say;"We believe," and will not be tested.
This is a rebuke in the form of a question, meaning that Allah will inevitably test His believing servants according to their level of faith, as it recorded in the authentic Hadith;
أَشَدُّ النَّاسِ بَلَءً الاَْنْبِيَاءُ ثُمَّ الصَّالِحُونَ ثُمَّ الاَْمْثَلُ فَالاَْمْثَلُ يُبْتَلَى الرَّجُلُ عَلَى حَسَبِ دِينِهِ فَإِنْ كَانَ فِي دِينِهِ صَلَبَةٌ زِيدَ لَهُ فِي الْبَلَء
The people most severely tested are the Prophets, then the righteous, then the next best and the next best. A man will be tested in accordance with the degree of his religious commitment; the stronger his religious commitment, the stronger his test.
This Ayah is like the Ayah,
أَمْ حَسِبْتُمْ أَن تَدْخُلُواْ الْجَنَّةَ وَلَمَّا يَعْلَمِ اللَّهُ الَّذِينَ جَـهَدُواْ مِنكُمْ وَيَعْلَمَ الصَّـبِرِينَ
Do you think that you will enter Paradise without Allah knowing those of you who fought (in His cause) and knowing those who are the patient. (3;142)
There is a similar Ayah in Surah At-Tawbah.
And Allah says;
أَمْ حَسِبْتُمْ أَن تَدْخُلُواْ الْجَنَّةَ وَلَمَّا يَأْتِكُم مَّثَلُ الَّذِينَ خَلَوْاْ مِن قَبْلِكُم مَّسَّتْهُمُ الْبَأْسَأءُ وَالضَّرَّاءُ وَزُلْزِلُواْ حَتَّى يَقُولَ الرَّسُولُ وَالَّذِينَ ءَامَنُواْ مَعَهُ مَتَى نَصْرُ اللَّهِ أَلَا إِنَّ نَصْرَ اللَّهِ قَرِيبٌ
Or think you that you will enter Paradise without such (trials) as came to those who passed away before you! They were afflicted with severe poverty and ailments and were so shaken that even the Messenger and those who believed along with him said, "When (will come) the help of Allah?"
Yes! Certainly, the help of Allah is near! (2;214)
Allah says here;
وَلَقَدْ فَتَنَّا الَّذِينَ مِن قَبْلِهِمْ فَلَيَعْلَمَنَّ اللَّهُ الَّذِينَ صَدَقُوا وَلَيَعْلَمَنَّ الْكَاذِبِينَ