Worshiping God is a spiritually good thing, right?First off, the Jews of St. Paul's day worshiped God, but not according to knowledge (see Romans 10:2).
Romans 3 says unregenerate man doesn't do it.
Secondly, St. Paul says that the Athenians worshiped God in ignorance: "What therefore you worship in ignorance, that I proclaim to you" (Acts 17:23).
So we have two examples where St. Paul himself affirms that God is worshiped in ignorance. That it is possible to worship in ignorance is also confirmed by Jesus himself, when he tells the Samaritan woman, "You worship what you do not know" (John 4:22).
Thirdly, not all worship of God is received with approval by God. This is evident from Isaiah 1:12-15, where he condemns the worship being offered by the Israelites. The fact that God rejected their worship does not change the fact that they offered it to him. We can see the same thing from the story of Nadab and Abihu, who offered up profane fire to God (Leviticus 10:1-5): this act of theirs was an act of worship, notwithstanding the fact that it was an improper act.
So we have two examples where God rejects worship that has been offered because of sin on the part of the worshiper. That fact does not change the fact that what was being offered was in fact worship.
I do not see how the situation is substantively different with respect to Muslims.
No Christian would deny that there are errors in Muslim beliefs about God - but the same is true with respect to the Jews in St. Paul's day, and it is simply not credible to suggest that they didn't worship the true God. So the fact of theological error does not invalidate what one claims about whom he worships. It means that he believes false things about the God he worships. That is a distinct proposition, however, from the question of whom he worships.
Secondly, even if we grant arguendo that the Jews' place as God's covenant people is so overwhelmingly significant that they can be said to worship the true God despite their errors while we at the same time deny that others do the same despite their errors - even if we grant this, the fact that St. Paul identifies the Athenians' Unknown God as our God means that it is certainly possible for anyone - even the Muslims - to worship God in ignorance.
I simply do not see what is controversial about this. It doesn't mean that the Muslims are Christians. It doesn't mean that they are saved differently than we are.
Perhaps part of the problem for some folks is that they have taken Romans 3 rather more literally than they should. I have addressed this before. In any case, I think that there is no good Scriptural reason not to take Muslims at their word about whom they claim as their God. This doesn't mean that Mohammed actually was a prophet. And (repeating myself) it doesn't mean that they are saved differently than we are.
It seems to me that those who deny this would need to be able to explain some things. If you say that only the regenerate can worship God, then you would need to be able to explain how the Jews of St. Paul's day worshiped God. Were they regenerate or not? If they were, how could this be since they denied that Jesus was the Messiah? If they did not worship the true God, then Romans 10:2 needs explaining. And St. Paul's interaction with the Athenians needs explanation too.
And from one man he has created the whole human race and made them live all over the earth, determining their appointed times and the boundaries of their lands; that they should seek God, and perhaps grope after him and find him, though he is not far from any one of us (Acts 17:26-27).It doesn't make sense, it seems to me, to suggest that Paul is talking about man before the Fall: man before the Fall would not have to "grope" after God, nor would they have to "seek" God: their fellowship with him would have been unbroken. No, clearly Paul has to be talking about man after the Fall. But this too suggests that it is by no means irrational to suggest that someone might worship God in ignorance.
In summary - let's take the Muslims at their word about whom they say that they worship, and pray for their salvation.