This is the first principle; doing actions sincerely for Allah the Blessed and Exalted, and warning and distancing oneself from Shirk. This is the principle which the Shaykh of Islam began with, may Allah have mercy upon him. This is the core of the foundation, the greatest of them, and the most beneficial of them without exception; making the actions sincerely for Allah, and warning against Shirk.
Making all actions sincere for Allah means: the worship of the person, and their obedience, to include prayer, Zakat, sacrificing, hope, fear, vowing, reliance, seeking help and other than that, must be sincerely for Allah the Blessed and Exalted.
The (action which is) sincere (ِِAl-Khaalis) is that which is pure and untainted; not containing impurity. It does not contain the impurity of Shirk, or the impurity of showing off or trying to be heard of, rather the person only seeks by his actions the Face of Allah the Blessed and Exalted. When he supplicates, he makes his Dua’a sincerely for Allah. When he relies, purifies his reliance only relying upon Allah, when he fears he purifies his fear, only fearing Allah. He performs all of his worship for Allah the Blessed and Exalted. He does not ask from other than Allah, he only seeks aid from Allah, he only puts his trust in Allah, and he entrust all of his affairs to Allah the Blessed and Exalted.