This "mystery" was already at work. Mystery refers to that which is concealed, hidden. This hidden, under the surface work, was now becomeing visible. Still mostly hidden, but enought was known about it to know it was already working.

Here the restrainer is considered a person. In verse 6 "that which" is present active neuter. Here it is masculine. This passage is difficult to put into English. It does seem clear the the evil was at work and would be of a limited duration. It was not yet fully revealed. Like the man of sin, there has been much discussion about the identity of the restrainer.

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Old Testament