The first time I dared to speak to my team about Agile and Scrum way of working it was appalling. It was during one of our retrospective meetings, at the time just called "team meeting". As good analytical people they are, all possible weaknesses and missing conditions were highlighted when I suggested to start working Agile.
They just concluded Agile was not possible because the organisation was not prepared and not willing to do it. They were of course right because we did not met all check boxes. Therefore I asked them what they considered was feasible at the time to implement.
If you want all or nothing with Agile, it could never start. There are various opinions on whether you can call Agile something that does not follow the "rules". Nevertheless, my aim was not to be recognised as an Agile pioneer, or receive a medal for implementing Agile in the company, I just believed in the overall principles and documented results in other organisations and wanted to get the benefits.
So my experience is that Agile starts with whatever your organisation and its leaders allow you to start with. Is like they say of babies: each baby let you be a parent (you cannot plan which type of parent you can be until you get to know your baby, right?). So How to "deal" with criticism? It is just feedback from your primary group of stakeholders. Dig on it, understand it, embrace it. What we could perceive as negativity or pessimism might only be constructive input expressed in a terrible manner.
If you are a believer, you just need to overcome emotional responses and try to remove specific barriers, starting with the people you trust the most. If you don't have a team yet, build one. Only after that you can really start receiving criticism and cynic comments from people that will help you grow.