The resignation letter from David Frost is available here on the BBC website and it makes clear several matters. One is that his resignation which he was not going to happen until January is now happening immediately. The reason for this is his private message to Johnson and his team was disclosed publicly by either Johnson or one of the his colleagues. Given that they announced it, he felt that he needed to depart straight away. That was a significant challenge for him. Then there is the text regarding Brexit. He begins the letter with the phrase

I have led our EU exit process for the two and half years since you became Prime Minister. In those years we have restored the UK’s freedom and independence as a country and begun the process of building a new relationship with the EU. 

Now most of us are aware that our independence was as clear prior to 2016 as it is now and indeed our Freedom is. Also the claim that our new relationship with the EU is moving forward is deeply questionable. However if we read through the letter to the beginning of the third paragraph he states

Brexit is now secure. The challenge for the Government now is to deliver on the opportunities it gives us.

So he is clearly admitting his and indeed our problems. What will be interesting to observe is how he will respond moving forward. Will he re-emerge in the House of Lords. If so will he support Johnson or be part of the people who want Johnson to disappear? No doubt we will discover that in 2022!