Quran

It is a known fact that Jews, Christians and hypocrites who live in Muslim societies used to correspond with their religious groups abroad to inform them of Muslims' plans and secrets. In one instance, this resulted in taking a group of Muslims as captives by Mongol Tatars. Ibn Taimyah: 2/496.
Question: It is forbidden to take Jews and Christians as allies. Why?

Love is the essence of alliance, and hatred is the essence of hostility. Love brings harmony and closeness while hatred causes dispersion and conflict. Ibn Taimyah: 2/498.
Question: What is the essence of alliance and that of hostility?

(So you see those with sickness in their hearts) such as doubt, hypocrisy and weakness of faith. They say: We take them as allies now because we may need them (We fear a misfortune may strike us) which means that in case things go in favor of the Jews and Christians, i.e. if they have the upper hand, and we would already be on good terms with them, then they will reward us for that. This is a negative way of thinking of Islam on their part. Allah says in response to their negative way of thinking, (Perhaps Allah will bring about victory) meaning that Allah may cause Islam to prevail. Al-Saadi: 235.
Question: Explain how negative way of thinking may lead to a great sin.

(Then they will regret) their friendship with the Jews and Christians which did not benefit them or protect them from any harm. Rather, it was nothing but harm, as Allah exposed their true reality to His faithful servants in this life, although they tried to conceal it. When the signs that exposed their hypocrisy were compiled against them, their matter became clear to Allah's faithful servants. So the believers were amazed at these hypocrites who pretended to be believers, swearing to their faithfulness, yet their claims were all lies and deceit. Ibn Kathir: 2/66.
Question: Whoever chooses to be an ally of the disbelievers, may be punished first in this life. Explain.

(They will be humble) due to their compassion and leniency toward the believers, and not humiliation; therefore it is a sign of strength. Al-Biqai: 2/483.
Question: What is meant by being humble toward the believers in this verse?

Being forceful and stern against the enemies of Allah is something that brings one closer to Allah, and is a means by which the person will be in harmony with his Lord in His wrath toward them. The idea of being stern toward them does not mean that we cannot call them to Islam in the manner that is best, thus combining sternness toward them with gentleness in calling them to Islam. Both are in their best interests and will benefit them. Al-Saadi: 236.
Question: When should one be stern against the disbelievers and when should he be gentle?

Having praised them for that which He blessed them with of noble and sublime characteristics, which are indicative of righteous deeds that are not mentioned, Allah then states that this is by His grace and kindness toward them, lest they be filled with self-admiration, and so that they would give thanks to the One Who blessed them with that. Allah, in turn, will bestow more of His grace upon them and others will understand that there is no barrier to the grace of Allah. Al-Saadi: 236.
Question: The characteristics of the believers are because of Allah's Grace upon them.