وَإِذَا قِيلَ لَهُمْ آمِنُوا بِمَا أَنزَلَ اللَّهُ قَالُوا نُؤْمِنُ بِمَا أُنزِلَ عَلَيْنَا وَيَكْفُرُونَ بِمَا وَرَاءَهُ وَهُوَ الْحَقُّ مُصَدِّقًا لِّمَا مَعَهُمْ ۗ قُلْ فَلِمَ تَقْتُلُونَ أَنبِيَاءَ اللَّهِ مِن قَبْلُ إِن كُنتُم مُّؤْمِنِينَ
Wa izaa qeela lahum aaminoo bimaaa anzalal laahu qaaloo nu'minu bimaaa unzila 'alaynaaa wa yakfuroona bimaa waraaa'ahoo wa huwal haqqu musaddiqal limaa ma'ahum; qul falima taqtuloona ambiyaaa'al laahi min qablu in kuntum mu'mineen
When they are told, "Believe in what God has sent down," they say, "We believe in what was sent down to us" — and they reject what came after, though it is the truth confirming what they already have. Say: "Why then did you kill the prophets of God in earlier times, if you were believers?"
The reply addresses a present group about a past act, which raised an old question about how blame carries across generations. Commentators generally answered that approval of a deed joins a person to it, and that the passage speaks to a community treated as continuous rather than to individuals. What triggers it is a formula of partial belief, accepting what came to us and refusing what came after.