Shaykh Ubayd al-Jaabiree (hafidhahullaah) was asked a question regarding differentiating between aqidah (creed) and manhaj (methodology):

Is there a difference between aqidah and manhaj

And he replied:

Aqidah is that which obligatory for a man to believe concerning Allaah the Mighty and Majestic, and that which has come from Him and concerning His Messengers and whatever His Messengers came with. And the pillar and summary of is the six pillars of Imaan which are Imaan in Allaah, His Angels, His Books, His Messengers, the Last Day and Imaan in al-Qadar, its good and its bad. Then whatever follows thereafter from that which is obligatory for a Muslim to believe, and that it is true and correct. Such as the affairs of the unseen like the emergence of fitan (tribulations) as was informed by the Prophet (sallallaahu 'alayhi wa Sallam), and the news concerning those who have passed of the Prophets and Messengers, regardless of whether that is in the Book or the Sunnah. Also the situations in the Barzakh, and the punishement and bliss of the grave, and what will take place on the major Judgement day, such as the placing of the Hawd, the Bridge, the Scales and what is besides that.

As for manhaj, then it is a corroboration, establishment of the usool of the deen and its furoo' (foundations and branches), the manhaj is the path by which a person corroborates the foundations and branches of the deen. So if this path is in agreement with the Book and the Sunnah and the way of the Salaf us-Saalih, then this is a true manhaj. And if it is in opposition to that then it is a corrupt manhaj.

And Islaam is brought together (composed) of these two, (a) the soundness, correctness in aqidah, and (b) the soundness and safety in manhaj. Therefore, one of them cannot be separated from the other.

The person whose manhaj is corrupt, have trust that this follows on from the corruption in his aqidah. When the aqidah is upright, in the correct way, then the manhaj will also be upright.

For the manhaj of the Khawaarij became corrupt because of the corruption in their aqidah. They held the belief of the lawfulness of the blood of those who commit major sins, and thus they permitted fighting and killing them and revolting against the sinful disobedient rulers. They made lawful the taking of blood and wealth. It is for this reason that it was said "They are disbelievers" by whoever said this from the people of knowledge (i.e. because they, the khawaarij made lawful what Allaah made unlawful, the taking of the blood of a Muslim upon other than a due right).

Source: The cassette (الإيضاح والبيان في كشف بعض طرائق فرقة الإخوان), "Elucidation and Clarification on Exposing Some of the Ways of the Sect of al-Ikhwaan [al-Muslimoon]".