The text of the chapter [from verse 6 to the end]
6. Indeed, they who disbelieved among the People of the Scripture and the polytheists will be in the fire of Hell, abiding eternally therein. Those are the worst of creatures.
7. Indeed, they who have believed and done righteous deeds — those are the best of creatures.
8. Their reward with Allah will be gardens of perpetual residence beneath which rivers flow, wherein they will abide forever, Allah being pleased with them and they with Him. That is for whoever has feared his Lord.
Allah, the Most High, informs us about the end of the unjust rejecters of faith from among the People of the Book and the polytheists who oppose the revealed Books of Allah and the Prophets, may Allah exalt their mention, sent by Him, saying that they (the rejecters) will be thrown into the Hell-fire on the Day of Resurrection, to abide therein for all eternity. They will not be able to evade it, nor will they be removed from it.
"Those are the worst of creatures."
That is, the most despised of Allah's creatures. Then He, Most High, informs us concerning those who are pious, fear Allah and are true Believers, confirming by their words and deeds that they are the best of created beings. Abu Hurayrah and a number of Muslim scholars have concluded from this verse that the true believer is superior to the angels, who are part of the creation.
"Their reward with Allah " that is, on the Day of Resurrection.
"Gardens of Eden under which rivers flow; they will dwell therein forever." That is, without interruption, without ceasing.
"Allah being pleased with them-" That is, Allah's Pleasure is better and more sublime than the lasting felicity, which they are given.
"and they with Him." That is, with the infinite Grace bestowed on them by Him.
"That is for whoever has feared his Lord."
That is, this reward can be achieved by him who fears Allah and is devout with a sincere devotion, who worships Him always as if he was seeing Him, knowing that, though he does not see his Lord, his Lord sees him. It is reported on the authority of Abu Hurayrah that he said: "The Messenger of Allah said:
"Shall I not inform you about the best of creation?" They (the audience) said: "Certainly, O, Messenger of Allah!" He said: "A man who takes the reins of his horse (and rides it) in the Way of Allah (in Jihaad) and whenever he hears the call of one in distress, he rides to his rescue. Shall I not inform you of the best of creation?" They said: "Certainly, O, Messenger of Allah!" He said: "A man who (even when he) is busy with his sheep, is steadfast in prayer and pays Zakah (alms). Shall I not inform you about the worst of creation?" They said: "Certainly, O, Messenger of Allah!" He said: "The one who asks in Allah's Name, but does not give in His Name." [Imam Ahmad]
Maalik Ibn 'Amr Ibn Thaabit Al-Ansaari, may Allah be pleased with him, said: "When the ayah (Quranic verse) "Indeed, they who disbelieved among the People of the Scripture and the polytheists…" [Al-Bayyinah] was revealed, Archangel Jibreel (Gabriel) said to Prophet Muhammad : "O, Messenger of Allah! Verily, your Lord orders you to recite this Soorah to Ubayy (one of the prominent reciters and scribes of the Noble Quran among the Companions)." So, the Messenger of Allah said to Ubayy : "Verily, Jibreel has ordered me to make you recite this Soorah."Ubayy said: "Then I was mentioned, O, Messenger of Allah?" He said: "Yes." Then Ubayy wept. [Imam Ahmad, Al-Bukhari and Muslim]
Imam At-Tirmithi and An-Nasa'ee have reported something similar. This Soorah was read to Ubayy Ibn Ka'b merely to strengthen him in his faith, to convey the Message and as a reminder, not for Ubayy to learn something which he did not know or to make him memorize it - and Allah Knows best.
Source: Tafseer Ibn Katheer, Juz' 'Amma