When Jesus appears the second time to the apostles and Thomss is present Jesus invites he to do what he said he needed to do in order to believe and then he gives him a comand
He commands him to change
And here we reach the binary decision. The English translations often say things like
Stop doubting and believe (NIV), or
Do not be faithless but believe (KJV) or
Do not be unbelieving but believe (NAB)
The first difficulty is that the verb is not 'to be' but it means to become or to continue. Being can be a static thing. We tend to think of being as rather fixed. The word John uses implies change, it comes from the same root as Genesis or generate.
Secondly John offers two choices pistos (to have faith) or apistos (the exact opposite). Unbelieving is probably the closest we have to an equivalent. It is more than doubt. Doubt is somewhere in the middle. For John there is no middle, you either believe or don't believe. You are either pistos or apistos.
Jesus calls Thomas to go from one state to the other, to become something different.
It would be great if this were a one time thing. But we human beings are constantly in flux, and if we look closely we will see that we vacillate between the two states. There are moments when we are sure, when we trust God, and then we switch. Our fears take over and we are back to depending not on God but on our own power. We go from the peaceful state of pistos to the frantic state of apistos.
The challenge is to recognize when the switch happens and to recognize it as quickly as possible. The faster we recognize it the faster we can get ourselves back to the other side. And when we can't get ourselves back to pistos, we can pray the prayer of Mark 9:24
I believe, help my unbelief (apistia)
This is the prayer of a man who understands his own vacillations.
If you are uncertain which state you are in simply ask, Am I peaceful? Am I content? When we truly trust Gos, when we are sure that God is Lord of all, we can as St. Paul says in his Letter to the Philippians, learn to be content in whatever circumstance we find ourselves.
As we walk through this Sunday and each day, let us regularly pause and check which state are we in pistos or apistos. It really is that simple. Either we trust God or we don't. It's not easy but it is simple.