أَنْزَلَ مِنَ السَّمَاءِ مَاءً فَسَالَتْ أَوْدِيَةٌ بِقَدَرِهَا فَاحْتَمَلَ السَّيْلُ زَبَدًا رَابِيًا وَمِمَّا يُوقِدُونَ عَلَيْهِ فِي النَّارِ ابْتِغَاءَ حِلْيَةٍ أَوْ مَتَاعٍ زَبَدٌ مِثْلُهُ كَذَلِكَ يَضْرِبُ اللَّهُ الْحَقَّ وَالْبَاطِلَ فَأَمَّا الزَّبَدُ فَيَذْهَبُ جُفَاءً وَأَمَّا مَا يَنْفَعُ النَّاسَ فَيَمْكُثُ فِي الْأَرْضِ كَذَلِكَ يَضْرِبُ اللَّهُ الْأَمْثَالَ ﴿۱۷﴾

﴾17﴿ Anzala minas samaaa'i maaa'an fasaalat awdiyatum biqadarihaa fahtamalas sailu zabadar raabiyaa; wa mimmmaa yooqidoona 'alaihi fin naarib tighaaa'a bilyatin aw mataa'in zabadum misluh; kazaalika yadribul laahul haqqa wal baatil; fa ammaz zabadu fa yazhabu jufaaa'aa; wa ammaa maa yanfa'un naasa fa yamkusu fil ard; kazaalika yadribul laahul amsaal

﴾17﴿ He sent down water from the sky, and the valleys flowed according to their capacity, and the torrent carried along a rising foam. And from what they heat in the fire to make ornaments or utensils arises a similar foam. Thus does Allah present the example of truth and falsehood. As for the foam, it vanishes as scum, but what benefits the people remains on the earth. Thus does Allah present examples

[17] In this verse, at the beginning, the eleventh rational argument is presented, followed by two examples that illustrate the contrast between truth and falsehood.
First Example:
When rain falls, streams and rivers swell with water, each according to its capacity. On top of the floodwaters, foam appears. After some time, the foam dries up and vanishes, while pure water remains, which people benefit from.
Similarly, the knowledge of monotheism (tawhid) and the Prophetic tradition (sunnah) is attained, and each person becomes knowledgeable according to the effort they make. Then, like the spreading floodwaters, this knowledge reaches various regions and people through da'wah (invitation to truth).
At first, people of falsehood may seem dominant, but after a while, they weaken and fade, and the true religion remains, from which people continue to benefit.
Second Example:
Goldsmiths melt gold, silver, iron, and tin in fire. From some of these, they make ornaments for beauty, and from others, useful tools and utensils. However, on the surface of the molten metal, scum or dross also appears, which later disappears, leaving behind pure and beneficial metal.
Likewise, scholars who, according to their ability, acquire the knowledge of the Qur’an and Sunnah, and strive in teaching and spreading it through lectures and lessons, separate false beliefs, polytheism, and innovations from the truth. Through this effort, other scholars arise, who are beneficial individuals, bringing benefit to others—or at the very least, benefit themselves through their knowledge.
In contrast, there are false scholars and misguided clerics who oppose them. After a while, these false claimants become weak and irrelevant, and the truthful ones continue to benefit the people.
Note: Since da'wah (invitation to truth) has two methods—one through traveling and preaching, and the other through teaching and education—therefore two examples are mentioned. But the foundation of both is reciting and remembering the Qur'an and Hadith, because without divine knowledge (revelation), there can be no true da'wah.