Based on the information provided, the suffix –ist here most likely means:
“one who practices”
Step-by-step explanation:
A suffix is/are a group of letters that are added towards the end of a noun or verb which would add more meaning to the word or also change the function of the word.
Regarding the same, the letters –ist is added to the word science which makes it “scientist” and similarly for atom, making it “atomists”. This changes the meaning of the word by making it belong to a person in the form of an occupation that a person holds.