The answer
We remember in the story of the righteous Job, that the Bible said: "One day the angels came to present themselves before the Lord, and Satan also came with them. The Lord said to Satan, 'Where have You come from?' " (Job. 1:6-7). And Satan plotted against Job.
So Satan was able to dare to stand in a holy place where God Himself stood, in order to try and cause to one of God's believers harm.
We read how Satan came to the Lord Christ on the mountain and dared to tempt Him, using verses from the Bible, and what
is more, he also stood with the Lord Christ on the pinnacle of the temple to put Him to the test there.
But of course that was all with the Lord's permission.
We hear in the Old Testament of sins that were committed in holy places, in the days of Eli the priest, by his sons which provoked God's anger. No doubt these were caused by Satan's intervention.
Satan might enter a church to distract the thoughts of the believers, to take their minds off of the prayers, out of envy that they are worshipping God.
And although some believers might defeat him by the strength of their prayers, others may be weakened. Whether a church has been consecrated isn't really the issue, because it depends on whether the individual believer has been consecrated, through being anointed with oil at baptism. Nevertheless, Satan can still enter his heart and thoughts to test him.
God may give Satan freedom to act, but it would be freedom within a limited sphere, and he would be judged for it.
So we say that Satan nowadays is fettered, and has been since the Crucifixion. And if Satan is fettered it means that he isn't totally free, otherwise he would have destroyed the world by now!
There have been times when the Lord has said: 'Go away Satan!', as happened at the temptation on the mountain, or when He has set him limits which he cannot exceed, such as in Job's temptation.
I am pretty sure that most of all Satan cannot bear the moment when the Holy Spirit descends and the sacraments are transformed during the divine Mass.
He cannot bear these holy moments, and God does not permit him to act then. Also, at this point, the believers are usually in an elevated spiritual state in which it is likely for them to respond to distracting ideas from Satan who at that moment is troubled by the deep heartfelt humility of the believers, and the action of the Spirit upon the sacraments and the congregation.
Generally speaking, if Satan enters a church in order to do something then he is in a weak position. And he cannot find real scope to act there except within the people who are inside the church, but whose hearts and minds are outside the Church!
Satan may try to cast doubts in people's minds, even on holy occasions and during prayers, but since the people's hearts are connected to God, any doubts that they might have remain outside them, however heavy and forceful they might be, and thus Satan has to depart unsuccessful.
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Thrice-Blessed Pope Shenouda III of Blessed Memory