DEFENDING THE RELIGION
Imaam Ahmad Ibn Hanbal (d.241H) ” rahimahullaah ” said: “All praise is due to Allaah who, in every age and interval between the Prophets, raises up a group from the people of knowledge, who call the misguided to guidance, patiently bearing ill-treatment and harm. With the Book of Allaah they give life to the dead and with the Light of Allaah they give sight to the blind. How many a person killed by Iblees (the Devil) they have revived. How beautiful their effect has been upon the people, and how vile the people have been towards them. They expel from the Book of Allaah the alterations of those going beyond bounds, the false claims of the liars and the false interpretations of the ignorant-ones ” those who uphold the banner of innovation and let loose the trials and discords; who upon the Book, oppose the Book and agree upon opposing the Book. They speak about Allaah and his Book without knowledge, argue about what is ambiguous in the Book, and deceive the ignorant ones with such ambiguities. We seek refuge in Allaah from the trials of the misguided ones.” 
Shaykhul-Islaam Ibn Taymiyyah (d.728H) ” rahimahullaah ” said: “When some people said to Imaam Ahmad Ibn Hanbal that they felt uneasy about criticizing people, he replied: “If I were to remain silent, how would the ignorant masses know the truth from falsehood?”
Those who introduce heretical writings which oppose the Qur`aan and Sunnah and those who innovate in matters of worship, then it is obligatory that they be exposed and that the Muslims be warned against them â€” by unanimous agreement of the Muslim scholars. In fact, when Imaam Ahmad Ibn Hanbal was asked about a man who fasted, prayed and secluded himself in the mosque for worship; if he was dearer to him than a person who spoke against the people of innovation (Ahlul-Bid’ah), he replied: “When he fasts and prays and secludes himself, then he does so for the benefit of his own self. However, when he speaks out against the innovators, he does so for the benefit of the Muslims in general, and this is more virtuous.”
So it is clear that openly opposing the innovators is of general benefit to the Muslims and is considered one of the types of jihaad (struggle) in the path of Allaah. Since purifying the Religion of Allaah and defending it from their attacks is a collective obligation ” as is agreed upon by the scholars. For if Allaah did not raise up some people to oppose the innovators, then the Religion would suffer harm, corruption and deviation. Indeed this type of corruption is even greater than the corruption resulting from the corruption of the disbelievers conquering the Muslims. Since when the unbelievers conquer the Muslims, they do not corrupt their hearts, nor their Religion, except after some time. Whereas the innovators corrupt the hearts from the very beginning.” 
UNITING THE RANKS UPON THE TRUTH
Shaykh `Abdul `Azeez Ibn Baaz (d.1420H) ” rahimahullaah” said: “There is no doubt that it is obligatory upon the Muslims to unify their ranks and to unite their word upon truth and to cooperate in goodness and piety against the enemies of Islaam ” as Allaah the Most Perfect ordered them with His statement: “And hold fast altogether to the rope of Allaah and do not become divided.” [Soorah Aali-`Imraan 3:102]
And likewise, Allaah has warned the Muslims against splitting-up, as occurs in His ” the Most Perfect’s saying: “And do not be like those who differed and split-up after the clear evidences came to them.” [Soorah Aali-`Imraan 3:105].
However, the order to unify the Muslims and to unite their word upon the truth and to hold fast to the rope of Allaah, does not necessitate that they should not censure wrong beliefs and practices whether from the Soofees or from other than them. Rather what the order to hold fast to the rope of Allaah necessitates is: to order the good, forbid the evil and also to clarify the truth with clear Sharee’ah evidences to whomsoever is misguided or has a mistaken opinion, until they unite upon the truth and turn away from that which opposes it. All of this is included in His the Most Perfect’s saying: “And help you one another in righteousness and piety, and do not help one another in sin and transgression.” [Sooratul Maa`idah 5:2]
And His the Most Perfect’s saying: “Let there arise from amongst you a group of people calling all that is good, enjoining the good and forbidding the evil, they are the ones who shall be successful.” [Soorah Aali-`Imraan 3:110]
And when the people of truth withold from clarifying the mistakes of those who have erred or are mistaken, then they will not have achieved what Allaah ordered them with, as regards calling to goodness, ordering the good and forbidding the evil. So the person in error will remain upon his error, and the person acting in opposition of the truth will remain upon his opposition. And this is contrary to what Allaah the Most perfect prescribed, with regards to sincere advice, cooperation upon goodness, ordering the good and forbidding the evil and Allaah alone is the One who grants success.” 
Shaykh `Abdul `Azeez Ibn Baaz was asked about the correctness of the statement: “We should unite upon that which we agree, and excuse each other for that in which we disagree.” The Shaykh commented: “Yes! It is obligatory to cooperate in that which we agree, by aiding the truth, calling to it and warning against that which Allaah and His Messenger (saw) have prohibited. As for excusing each other for that in which we differ – then this is not to be taken in an absolute sense, but rather in a general one. Thus if the difference involves a particular issue of ijtihaad (independent reasoning) wherein the proofs are deep and detailed then it is obligatory not to censure or reproach one another. However, if it opposes a clear text of the Book and the Sunnah, then it is obligatory to correct the one who has opposed the clear texts, but with wisdom, beautiful admonition, and debating in the best possible manner, acting upon the statement of Allaah the Exalted: “And help you one another in righteousness and piety, but do not help one another in sin and transgression.” [Sooratul Maa`idah 5:2]
And His the Most Perfect’s saying: “The Believers men and women are protectors to each other, they order the good and forbid the evil” [Sooratut-Tawbah 9:71]
And the statement of Allaah the Mighty and Majestic: “Invite to the way of your Lord with wisdom and beautiful admonition, and argue with them in ways that are better.” [Sooratun-Nahl 16:125]
And he (saw) said: “Whosoever sees an evil, then let him change it with his hand. If he is unable to do this, then with his tongue, and if he is unable to do this, then with his heart. And that is the weakest of eemaan (faith).” 
And he (saw) said:”Whosoever directs a person to do good, will have a reward similar to the one who does that good.” 
And the aayaat and ahaadeeth with this meaning are plenty.” 
-  ar-Radd `alal Jahmiyyah waz-Zanaadiqah (p. 2) of Imaam Ahmad Ibn Hanbal.
-  Majmoo’ul Fataawaa (28/231-232).
-  Tanbeehaat fir-Radd `alaa man Ta`wwalis Sifaat (p. 31-32).
-  Related by Muslim (2/21), from Aboo Sa’eed al-Khudree (raa).
-  Related by Muslim (13/38), from Aboo Mas’ood al-Ansaaree (raa)
-  Tanbeehaat fir-Radd `alaa man Ta`awwalis Sifaat (p. 14-15).