Hawzah News Agency (Qom, Iran) – It must be understood that there is no contradiction between 'isma and the free will of the ma'sum (the inerrant). Rather, a ma'sum, despite having perfect consciousness of God, and of the effects of obedience and disobedience, does have the power to commit a sin, even if he would never avail himself of this power. It is akin to the case of a kind father, who has the power to kill his own child, but would never do so.
A clearer instance of this principle is the non-existence of evil acts issuing from God. In His absolute omnipotence, God has the power to put pious souls in Hell and, inversely, disobedient souls in Heaven, but His justice and wisdom preclude such acts.
From this point it should be clear that the renunciation of sin and the accomplishment of worship and obedience are considered a great honour by the ma'sum; so, despite having the power to sin, the ma'sumin would never in fact commit one.
Ayatollah Jafar Sobhani, Doctrines of Shii Islam, A Compendium of Imami Beliefs and Practices, Translated and Edited by Reza Shah-Kazemi, published by I.B.Tauris Publishers, London • New York 2003.