The answer is: If being within his power means that the faculties by which the servant can act are sound, his limbs are healthy, his powers are intact, the means of action are available to him, and the way to act has been prepared and opened for him, then yes: it is within his power in this sense. But if it means the capacity that accompanies the act and necessarily brings it about, such that whenever this capacity exists the act invariably follows, then it is not within his power in this sense.
To explain: capacity is of two kinds. The first is enabling capacity: the availability of the means and conditions, together with sound faculties. This is the basis of religious obligation. It precedes the act but does not necessitate it. The second is capacity that accompanies the act and necessarily entails it, such that the act invariably follows. This is not a prerequisite for religious obligation; its validity and propriety do not depend upon it.
Thus, faith on the part of someone whom Allah has not willed to believe, and obedience on the part of someone whom He has not willed to obey, are within that person’s power in the first sense, but not in the second.
This distinction resolves the difficulty concerning the imposition of obligations beyond one’s capacity, as will be explained in its proper place, if Allah, the Exalted, wills.
If it is asked, “Has He created the capacity to believe in someone whom He knows will not believe, or has He not created such a capacity in him?”
The answer is: He has created in him an enabling capacity that precedes the act and forms the basis of commands and prohibitions. But He has not created in him a capacity that necessitates and entails the act, such that the act invariably follows. The latter is His bounty, which He grants to whomever He wills; the former is His justice, by which His proof against His servant is established.
If it is asked, “Can he then perform the act when this capacity has not been created in him?”
The answer is: This is precisely the same question as before, and you have already learned its answer. Success comes only through Allah.
Allah, the Exalted, says (5:3):
“Today I have perfected your religion for you, completed My favour upon you, and approved Islam for you as your religion.”