Beale lists like 8 possibilities based on the grammar; I think the interpretation that it is a Roman empire is wrong. Other possibilities are the gospel, Satan, or an angel for example. Just because a view is historical, doesn't make it correct. It could be both the gospel proclamation and the Holy Spirit since the Spirit inspires the scriptures.
2 Thessalonians 2:7 (AMP): For the mystery of lawlessness (that hidden principle of rebellion against constituted authority) is already at work in the world, [but it is] restrained only until [a]h...
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