Valid political structures (governments) exist to coordinate social activity to the benefit of the participants. The impetus on the individual is external and involuntary. It exerts it's force over you, and you become a small part of its totality. It is the framework you must observe and comply with to interact socially. It seeks to control, direct, and correct behavior physically. Your life is lived under it.
If one embraces, internalizes, and acts based on the dictates of what is in reality a political system, then to such a one, Caesar is lord.
Peter and the other apostles replied: "We must obey God rather than men!
Valid religion should of its nature help to bring one closer to God, and to each other, in truth and love to the benefit of all. The religious impetus for individual action is internal and voluntary, and the locus of control is within the individual. It is freely assented to, and given. It empowers, and drives thoughts, words, and actions, and colors behavior predicatably. It recognizes, and accomodates for the weakness of ones humanity, and provides the framework one uses to view and interact willfully in the world. It becomes part of what, and who you are. You live through it, with it, and in it: not under it.
Since true religions very purpose is to lead to that which is all-good, it follows that true Religion is always good. Fake religion maybe not so; but true religion must always be good in order to legitimately qualify as a contender for the path to all-goodness.
By their fruit you will recognize them. Do people pick grapes from thorn bushes, or figs from thistles?
Looking at the fruit of Islam (from my POV - outside of it), I see the attributes of both government and religion. It is observable that it takes power from the individual, imposes behavior, requires compliance, and exerts physical penalties. The motive of Islam appears to be mostly to benefit its participants. Islam also displays a this-worldy orientation in its stated goal of imposing itself on all via influence, hegemony, or force if required. That is a purely political ambition if you look at it honestly.
It appears that the labelling of Islam as a religion could be akin to wrapping what is really a political system in a sort of sheep's clothing. In doing so people inside it will tend to comply and revere it, and those outside of it will give it the courtesy of religious tolerance and the consequent freedom to act relatively unimpeded among the people. But remember that painting the bark on a tree, does not change it's fruit. You must look at the fruit to discern the tree.
Can Islam be both a religion and a political system?
"No one can serve two masters. Either he will hate the one and love the other, or he will be devoted to the one and despise the other. You cannot serve both God and Money."
So which is it? a religion, or a political system? Based on the observable evidence, Islamism does qualify as a political system, so is it therefore invalid to blame or credit "religion" for Islamic acts? Does Islam serve Caesar, while wrapped in the garb of religious service to Allah?
"There is no possible compromise with it and he must either convert to some happier faith, move to a state which agrees with his moral vision, become a hypocrite to his own ideals, or find himself at war with us."
"But the reality is that Islam is an invented human religion that borrows from fragments of Judaism and Christianity, mixes in Mohammed's own claims of revelation, and completes it with a dash of conventional wisdom from seventh-century Arab culture. It is not a magisterial faith with some adjudicating body that defines what is and is not the orthodox reading of the Koran. It is whatever its various adherents say it is"
"They struck the Mediterranean world at a time when domestic strife and war made a common front against Arab Muslim expansion impossible."
"...this is the ambiguity of Islam, from its beginning to our present day: violence is a part of it..."