Since Allah is a god who lets people off the hook if they try to keep the five pillars of Islam, he cannot be just. The Christian God is a just God because he is the essence of all that is good, and "goodness" itself flows from his character and is defined by Him. Therefore, as Jonathan Edwards explains in his classic, "The Justice of God in the Damnation of Sinners", there is no way sinful man can ever be right with God, because man is as sinful as God is infinitely good:

But God is a being infinitely lovely, because he hath infinite excellency and beauty. To have infinite excellency and beauty, is the same thing as to have infinite loveliness. He is a being of infinite greatness, majesty, and glory; and therefore he is infinitely honourable. He is infinitely exalted above the greatest potentates of the earth, and highest angels in heaven; and therefore he is infinitely more honourable than they. His authority over us is infinite; and the ground of his right to our obedience is infinitely strong; for he is infinitely worthy to be obeyed himself, and we have an absolute, universal, and infinite dependence upon him.

So that sin against God, being a violation of infinite obligations, must be a crime infinitely heinous, and so deserving of infinite punishment.- Nothing is more agreeable to the common sense of mankind, than that sins committed against any one, must be proportionably heinous to the dignity of the being offended and abused; as it is also agreeable to the word of God, I Samuel 2:25. "If one man sin against another, the judge shall judge him;" (i.e. shall judge him, and inflict a finite punishment, such as finite judges can inflict;) "but if a man sin against the Lord, who shall entreat for him?" This was the aggravation of sin that made Joseph afraid of it. Genesis 39:9. "How shall I commit this great wickedness, and sin against God?" This was the aggravation of David's sin, in comparison of which he esteemed all others as nothing, because they were infinitely exceeded by it. Psalm 51:4. "Against thee, thee only have I sinned."-The eternity of the punishment of ungodly men renders it infinite: and it renders it no more than infinite; and therefore renders no more than proportionable to the heinousness of what they are guilty of.

If there be any evil or faultiness in sin against God, there is certainly infinite evil: for if it be any fault at all, it has an infinite aggravation, viz. that it is against an infinite object. If it be ever so small upon other accounts, yet if it be any thing, it has one infinite dimension; and so is an infinite evil. Which may be illustrated by this: if we suppose a thing to have infinite length, but no breadth and thickness, (a mere mathematical line,) it is nothing: but if it have any breadth and thickness, though never so small, and infinite length, the quantity of it is infinite; it exceeds the quantity of any thing, however broad, thick, and long, wherein these dimensions are all finite.

So that the objections made against the infinite punishment of sin, from the necessity, or rather previous certainty, of the futurition of sin, arising from the unavoidable original corruption of nature, if they argue any thing, argue against any faultiness at all: for if this necessity or certainty leaves any evil at all in sin, that fault must be infinite by reason of the infinite object.