Greta sails in a high-tech boat to a UN meeting against climate change. This luxury (and obviously exciting) adventure causes more GHG emissions than a business flight, since the crew of the race boat has to fly intercontinentally. More importantly, there is no need to physically travel to such meeting, since there is internet. A saint would be supposed to show this by example. Rather than that, Greta has become the Mascot of the movement and her arrival in New York almost looked like the Pope visiting a catholic country.
The case shows the issue that is common in religions: preaching water and drinking wine. And it is even more common in climate discussions. Of course, there is always an excuse why a travel is “needed”. I observe it a lot in science, where conference attendance and other meetings cause enormous greenhouse gas emissions through air travel. Of course, all these scientists are so important for the cause and thus the emissions can be justified - the same applies to Greta or business travelers...
It is always easy to blame the others for cuausing climate change, as Germany and Norway do in the case of Brazil and its Amazon fires. However, Norway, a major crude oil producer (top 6 per-capita), and Germany, a major coal producer, could contribute their share by simply stopping these climate hurting activities – without harming their economy too much since they are rich anyway.
Why don’t we just all reduce our consumption substantially? Living on rice and pulses might not be fun for everyone. Staying home without travel is boring for others. Demanding change for the society is easy. However, the effect of climate demonstrations and other political actions can be doubted, especially since in democratic countries we can be confident that drastic costs for climate action won’t pass parliaments or public votes, as it is non-liberal and inconvenient.
In the end, it does not matter what one believes, it matters how you act. Effective combat of climate change includes reducing own consumption, especially in areas of high impact, like traveling, food and housing. However, sins are often attractive and violating the rules for a better world might be joyful. Reducing climate change involves self-limitation, which means fighting basic instincts.
In general, believers are no saints. They might try, but they only achieve part of their goals. This is the real life and it is OK. However, we should all try to contribute our share, but without going into clashes of religions and cultures. So don't give up, and start sweeping around your own front door first!