In the Name of Allah, the Most Beneficent, the Most Merciful

1. Those who disbelieved among the People of the Scripture and the polytheists were not to be parted [from disbelief] until there came to them clear evidence —
2. A Messenger from Allah, reciting purified scriptures.
3. Within which are correct writings [i.e. rulings and laws].
4. Nor did those who were given the Scripture become divided until after there had come to them clear evidence.
5. And they were not commanded except to worship Allah, [being] sincere to Him in religion, inclining to truth, and to establish prayer and to give Zakah (alms). And that is the correct religion. [Quran 98:1-5] 
As for the People of the Scripture they are the Jews and Christians, while the polytheists are those who worship idols and fire from amongst the Arabs and the non-Arabs. Mujahid  said: "were not to be parted" means that they (the People of the Scripture) will not cease until the Clear Evidence came to them. What the 'Clear Evidence', is explained by the following verse (which means): "A Messenger from Allah, reciting purified scriptures",That is, Muhammad  and that which he recites from the Noble Quran, which was copied by the Heavenly Host (i.e., the angels), on purified sheets, as Allah Says: (what mean):
"[It is recorded] in honored sheets, exalted and purified, [carried] by the hands of messenger-angels, noble and dutiful." [Quran 80:13-16]
"Within which are correct writings [i.e., rulings and laws]."
Ibn Jareer  said: "It means that amongst the purified scriptures are Scriptures from the Books of Allah, of inestimable value, full of justice, perfectly correct, without any error, because they are from Allah, the Almighty, the All-Powerful."
Qatadah  said: "A Messenger from Allah, reciting purified scriptures" means that the Quran is mentioned with the best description and that Allah extols it in the most laudatory terms."
Ibn Zayd  said, concerning the verse (which means): "Within which are correct writings": "It means correct and fair. I say: The Words of Him, the Most High: "Nor did those who were given the Scripture become divided until after there had come to them clear evidence " are like His Words (which mean): "And do not be like the ones who became divided and differed after the clear proofs had come to them. And those will have a great punishment." [Quran 3: 105] That is, the communities (the People of the Books) before us, after Allah, the Exalted, revealed to them proofs and clear signs, became divided and disputed amongst themselves as to what was required of them by Allah, the Most High, in their revealed books. They became involved in major differences as is made clear by the Hadeeth (narration) in which the Prophet  said: "Verily, the Jews are divided into seventy-one different sects, and verily, the Christians are divided into seventy-two sects, and verily, my community will be divided into seventy-three different sects - all of whom will be in the Hell-Fire except one." His Companions asked: "And who are they, O, Messenger of Allah?" He  replied: "Those who follow the path of me and my Companions."
And they were not commanded except to worship Allah, [being] sincere to Him in religion, inclining to truth…"This is like Allah's Words (which mean):"And We sent not before you any messenger except that We revealed to him that, there is no deity except Me, so worship Me." [Quran 21: 25] For this reason, Allah Says (what means): "inclining to truth" that is, professing and practicing the True Religion (Islamic Monotheism) without associating any partners with Allah, as in His Words (which mean):"And We certainly sent into every nation a messenger, [saying] 'Worship Allah and avoid false deities.'" [Quran 16: 36]
"and to establish prayer"- which is the most noble of all physical activities.
"and to give Zakah (alms)" - which is to be charitable towards the poor and the needy.
"And that is the correct religion."
That is the True, the Correct, and the Just Faith - to belong to the most upright, religious community - a moderate community. Many scholars, including Az-Zuhri and Ash-Shafi'i  have concluded from this verse that true faith must include action (not just belief in the heart), and for this reason, Allah, the Exalted, Says (what means):
"And they were not commanded except to worship Allah, [being] sincere to Him in religion, inclining to truth, and to establish prayer and to give Zakah (alms). And that is the correct religion. "

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.

Explanation

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