In verse 9:105 who is "the believers" referring to ?
Is it referring to all the believer/Muslim or is it referring to some specific people?
And briefly what is the significance of "the believers" in the context of this verse?
To give you some context before answering your question:
Now to answer your question:
None of the tafsirs above mention specifically who are the believers, I guess because the definition of the believers exist in other verses which explain who is considered a believer. However,Baghawi in his tafsir indirectly gives the significance of the believers here:
believers were mentioned in this verse in order to witness the deeds of other people, Hence, the love of the believers toward those who have good deeds will increase (purpose of this is to motivate them to do more good deeds) and vice versa.