The close of probation is a concept that tends to strike fear in most Adventists and causes other Christians to view us with suspicion. But, as usual, it is because the concept is misunderstood.
All Christians believe there is a close of probation, even though they don’t use the phrase. This might be at the second coming or after the millennium, but there is a point when people no longer switch sides.
Unlike other Christians however, Adventists believe this event happens a short period of time before the second coming. And, many Adventists think this means that there will come a day when God will close the door of mercy and, even if they want to repent, God will no longer forgive them.
In chemistry, there are certain substances called catalysts which increase the rate of a chemical reaction. So if we mix together two substances and find that it normally takes several hours for the reaction to occur, adding a catalyst would reduce the time to several minutes instead.
Throughout every individual’s life, a decision is made whether or not to follow Christ. However, at any given time, you have three categories of people: those who chose Christ, those who rejected Christ, and those who are still on the fence. At some point, even those on the fence will make a decision, but, by then, more people will have been born and there will be a new generation of fence sitters.
Before the second coming however, God wants every one to make a decision for or against Christ. He wants to eliminate the fence sitter group. And to do this, God introduces a catalyst. This catalyst is a combination of the church proclaiming/demonstrating a powerful last day message but also of natural disasters, political/economic upheaval and other factors. Through all these things God will speed up the decision making process so that everyone makes up their mind whom they will serve.
Probation will close not to prevent people from any longer changing their mind, but BECAUSE people are no longer changing their mind. In other words, as a result of the catalyst, everyone has made their decision and there is no point in keeping probation open any further.
The reason probation closes before the second coming and not AT the second coming is because God wants to allow time to pass so that it is evident to angels and unfallen beings that people have made up their minds and are no longer going to change. Some period of time passes where, as much reason as God gives people to repent, no one does anymore. And then, heavenly beings all agree that there is no point in waiting any further and it’s time for Jesus to return.