The Quran

إِذ جاءَتهُمُ الرُّسُلُ مِن بَينِ أَيديهِم وَمِن خَلفِهِم أَلّا تَعبُدوا إِلَّا اللَّهَ ۖ قالوا لَو شاءَ رَبُّنا لَأَنزَلَ مَلائِكَةً فَإِنّا بِما أُرسِلتُم بِهِ كافِرونَ

Muhammad Asad

Lo! There came unto them [God's] apostles, speaking of what lay open before them and what was [still] beyond their ken,¹ [and calling unto them,] "Worship none but God!" They answered: "If our Sustainer had willed [us to believe in what you say], He would have sent down angels [as His message-bearers]², As it is, behold, we deny that there is any truth in what you [claim to] have been sent with!"

Lit., "from between their hands and from behind them": i.e., reminding them of something that was known to them - namely, what happened to sinners like themselves who lived before their time - and warning them of what was bound to happen in the future to them, too, if they persisted in their denial of the truth (Al-Hasan al-Basri, as quoted by Zamakhshari). However, it is possible to understand the above phrase (which has been explained in note 247 on 2:255) in yet another, more direct way: God's message-bearers pointed out to those sinning communities something that should have been obvious to them (lit.,"between their hands") - namely, their patently wrong attitude in their worldly, social concerns and moral concepts - as well as the unreasonableness of their denying something that was still beyond their ken (lit., "behind them"): namely, life after death and God's ultimate judgment.
Cf. 6:8-9 and 15:7