A good question
Why did God allow the Romans to feed Christian to the lions too?
Why did God allow the sacking and destruction of nearly all churches in the USSR?
And the funny thing is...both the Romans and the USSR are now gone - but Christianity remains.
Christians not know the plan. We just know the outcome. But it is not based on world conquest at the point of a sword...
But Christianity continued to grow in Rome despite the oppression. The Roman government was very desultory in its persecutions, and Christianity steadily grew until it took over the Empire. One might say that God judged pagan Rome, and gave the laurel of victory to the Christians.
The USSR was defeated, and there is some resurgence of Christianity in Russia. Some. (Russia still has an imploding birth rate and massive violence and corruption, and has in no real sense returned to being a "Christian country". It is now a country where Christianity is not officially suppressed. Also, the USSR failed within a couple of generations. Assuming Christianity does re-establish itself broadly in Russia, one might again say that, after a three-generation Communist hiatus, God gave the victory to the Christians.
But Islam...Islam took over half of Christian lands well over 1000 years ago. And Christianity died out almost completely in all of them. It was only in the Catholic West, particularly Iberia, where Islam was pushed back out, for a time. Today, Europe is more atheistic and secular than Christian, and Islam is the surging, vibrantly growing religion there.
Christianity defeated paganism in Rome and, perhaps, Communism in Russia, and this may have been the judgment of God.
But Islam defeated Christianity in the Middle East and North Africa, and is rapidly overtaking Christianity in Europe (where Christianity remains, but as a weak tradition versus a strong and growing Islamic pressure). Indeed, Christian Europe looks an awful lot like pagan Rome versus the Christians, but without any of the violent killings that did drive Christianity underground for a time in the Roman era. Might one see, too, the judgment of God in the apparently permanent victory of Islam over most of Eastern Christianity, and the steady and apparently inexorable advance of Islam against Western European Christianity (what's left of it)?
I'm not advocating that position, particularly.
But it is a serious question that I would like to see answered theologically.