The Quran

سَنُريهِم آياتِنا فِي الآفاقِ وَفي أَنفُسِهِم حَتّىٰ يَتَبَيَّنَ لَهُم أَنَّهُ الحَقُّ ۗ أَوَلَم يَكفِ بِرَبِّكَ أَنَّهُ عَلىٰ كُلِّ شَيءٍ شَهيدٌ

Muhammad Asad

In time We shall make them fully understand¹ Our messages [through what they perceive] in the utmost horizons [of the universe] and within themselves,² so that it will become clear unto them that this [revelation] is indeed the truth [Still,] is it not enough [for them to know] that thy Sustainer is witness unto everything?³

Lit., "We will show them" or "make them see".
I.e., through a progressive deepening and widening of their insight into the wonders of the umverse as well as through a deeper understanding of man's own psyche - all of which points to the existence of a conscious Creator.
I.e., that He is almighty and all-seeing: a fundamental truth which, by itself, should be enough to remind man of his responsibility before Him.