For years, I held this passage references three time frames: three events. Here’s what it looks like as a timeline:
First is the cross, which is the offering whereby Christ supplied the provision – the only basis for salvation. This is the “by one offering”. The third event is happening while the book of Hebrews is written; the sanctification of the believers. This is referenced by “are being sanctified”. In between is a second, implicit event; the conversion of the people spoken of in Hebrews; the moment when they first came to faith. This is when Christ “perfected forever” those believers. So the perfecting is in the past, relative to the writing of the book of Hebrews and is based on the cross.
Lately, I have been thinking about another explanation. Perhaps the passage references two events: the cross and those being sanctified at the time of the writing of Hebrews. Literally, the passage is saying we were perfected forever at the cross. However, the statement is a synecdoche, a part referenced as a whole. Christ is so certain that what He has done will perfect forever the people spoken of in Hebrews, that He can say, “I perfected them”. However, He must still intercede for them, He must still justify them; He must still sanctify them – and this He does only while they believe. And this certainty is primarily in knowing He has already done the hard part; He died for them. Secondary, this certainty based in His knowing the efficacy of His blood; whoever He applies it to will be cleansed.
There’s a great scene in Hunt for Red October involving two subs play chicken where one sub ends up torpedoing itself. When an officer on that sub learns his own torpedo is heading straight for him, he says to his commander “You’ve killed us.” Maybe in a similar sense, Christ has perfected whoever will believe.