“It is treading the straight path, -which is the upright religion – without diverting away from it; not to the right and not to the left. And this (treading the straight path) encompasses the acts of obedience,all of them; the outward and the inward and likewise it encompasses the abandoning of the prohibited matters in their entirety.”
“The outward and inward”; so all of the affairs of the religion are gathered and nothing at all is left.
Actions of the heart such as Eemaan, Love, Fear, Hope and the outward actions such as actions of the limbs (eg the Salah) and actions of the tongue (eg the Shahaadah).
There is nothing that is left from the religion of Allah. So commanding with istiqaamah is commanding with establishment of the affairs of the religion in entirety; outwardly and inwardly.”
Slightly Paraphrased from:
المجموع الرائق من الوصايا و الزهديات و الرقائق ص ٩٦