Matthew 26:56, it's probably mentioned in the other 3 books too.
Peter denied him 3 times, if you don't know that one I'm not bothering looking it up
They also needed visual proof of the resurrection, again if you don't know that I really question your study.
Ah I see what you're referring to and yes I'm quite familiar with all three passages of scripture.
So what we need here is context because a text without context is a pretext.
The difficulty I sometimes have is how to very succinctly unravel arguments such as these. Oh I can certainly unravel them, but doing so in a way that is succinct yet thorough enough is hard because GB readers (including myself) have a limited capacity for what they're willing to read and digest. It's super easy to lay down an argument or objection without doing your homework and walk away. It's an altogether different circumstance to defend against that argument/objection in quick and concise manner. It's still extremely possible to do so.
That said, we go back to the original premise which was that the 12 disciples hearts were closed (or hardened) and they lost faith.
The chain of logic is essentially the following: because Peter denied Christ 3 times, because Thomas required visual proof of the resurrection and because the apostles fled when Christ was captured by the Romans (on behalf of the Sanhedrin) that their hearts were closed/hardened and they lost faith.
So first things first, this logic is both an example of special pleading and a nonsequitur. Why? Because context is king and missing completely.
Second, what is meant by a hardened or closed heart? The Koine Greek defines the hardening of the heart as a callousing of the heart in which permanent denial of God has occurred. The most notable example of that being the pharaoh of Egypt.
That said, you simply can't justify your premise with the examples provided as valid justification because the apostles all went to their deaths evangelizing the gospel message of Jesus Christ to many nations (Judas excluded). Many of the apostles suffering brutal deaths for the cause of Christ.
Now, what's significant about historical context in this situation?
The Israelites/Jews had prophecy established hundreds of years prior to the birth of Christ. Their expectation of the coming Messiah was that of a conquering king that would destroy the enemy nations of Israel. Many expected the Messiah to arrive and destroy the Roman government. Unfortunately the Jews errored in interpretation of prophetic scripture and ignored the portions of scripture noting the gentleness by which the Messiah would arrive. Instead they focused on the conquering, vanquishing king to come and had difficulty accepting the meekness and humility of Christ.
Why did Peter deny Christ and why did the apostles flee when Christ was captured by Roman soldiers (on behalf of the Sanhedrin)? Well, despite all the miraculous situations they experienced while in the company of Christ and the divine revelations and teachings they received they were still overcome by fear when "things got real". They are human beings after all and fear can cause any of us to shrink at times.
Remember, believers today have the luxury of looking back at established history and drawing upon centuries of theological teaching and the ability to study fully canonized scripture. Believers today also have the definite guidance of the Holy Spirit that indwells them. The apostles were experiencing all of this unprecedented activity live and in vivid color. The Holy Spirit was not in them yet and they were afraid in those difficult moments and they errored in their ways.
Now, the infamous "doubting Thomas" story has NOTHING to do with a hardened/closed heart, but certainly does illustrate a lack of faith. God has promised that our faith will be repeatedly tested and as we become stronger and stronger believers that rate of success with increase. Again, everything that was being experienced live at that time was unprecedented and documented only via prophetic scripture which not all Jews had access to. The Pharisees and Sadducees of the Sanhedrin were the keepers and interpreters of this scripture.
So there were definitely moments in which the apostles faith was tested and in some instances they failed, but that has nothing to do with a closed/hardened heart. And it certainly doesn't make your argument work in anyway whatsoever.....unless you ignore context. I can't overcome that.