Allah said,

[قَـتِلُواْ الَّذِينَ لاَ يُؤْمِنُونَ بِاللَّهِ وَلاَ بِالْيَوْمِ الاٌّخِرِ وَلاَ يُحَرِّمُونَ مَا حَرَّمَ اللَّهُ وَرَسُولُهُ وَلاَ يَدِينُونَ دِينَ الْحَقِّ مِنَ الَّذِينَ أُوتُواْ الْكِتَـبَ حَتَّى يُعْطُواْ الْجِزْيَةَ عَن يَدٍ وَهُمْ صَـغِرُونَ ]

(Fight against those who believe not in Allah, nor in the Last Day, nor forbid that which has been forbidden by Allah and His Messenger, and those who acknowledge not the religion of truth among the People of the Scripture, until they pay the Jizyah with willing submission, and feel themselves subdued.) Therefore, when People of the Scriptures disbelieved in Muhammad , they had no beneficial faith in any Messenger or what the Messengers brought. Rather, they followed their religions because this conformed with their ideas, lusts and the ways of their forefathers, not because they are Allah's Law and religion. Had they been true believers in their religions, that faith would have directed them to believe in Muhammad , because all Prophets gave the good news of Muhammad's advent and commanded them to obey and follow him. Yet when he was sent, they disbelieved in him, even though he is the mightiest of all Messengers. Therefore, they do not follow the religion of earlier Prophets because these religions came from Allah, but because these suit their desires and lusts. Therefore, their claimed faith in an earlier Prophet will not benefit them because they disbelieved in the master, the mightiest, the last and most perfect of all Prophets . Hence Allah's statement,

[قَـتِلُواْ الَّذِينَ لاَ يُؤْمِنُونَ بِاللَّهِ وَلاَ بِالْيَوْمِ الاٌّخِرِ وَلاَ يُحَرِّمُونَ مَا حَرَّمَ اللَّهُ وَرَسُولُهُ وَلاَ يَدِينُونَ دِينَ الْحَقِّ مِنَ الَّذِينَ أُوتُواْ الْكِتَـبَ]

(Fight against those who believe not in Allah, nor in the Last Day, nor forbid that which has been forbidden by Allah and His Messenger, and those who acknowledge not the religion of truth among the People of the Scripture,) This honorable Ayah was revealed with the order to fight the People of the Book, after the pagans were defeated, the people entered Allah's religion in large numbers, and the Arabian Peninsula was secured under the Muslims' control. Allah commanded His Messenger to fight the People of the Scriptures, Jews and Christians, on the ninth year of Hijrah, and he prepared his army to fight the Romans and called the people to Jihad announcing his intent and destination. The Messenger sent his intent to various Arab areas around Al-Madinah to gather forces, and he collected an army of thirty thousand. Some people from Al-Madinah and some hypocrites, in and around it, lagged behind, for that year was a year of drought and intense heat. The Messenger of Allah marched, heading towards Ash-Sham to fight the Romans until he reached Tabuk, where he set camp for about twenty days next to its water resources. He then prayed to Allah for a decision and went back to Al-Madinah because it was a hard year and the people were weak, as we will mention, Allah willing.