We only sync new data from Mailchimp when you tell us to. You can choose "resync recent changes" in the integrations page (recommended), or choose to resync lists or contacts separately.
You also have an option to manually update a specific group you are working with by clicking on the resync icon within the groups page, or by clicking the resync icon in the group details page.
Another option if you want PastorsLine to automatically sync overnight is to turn "on" overnight syncing. This will automatically resync your account overnight. (This option is not available for all plans. You can check if it is available for you in your integrations page). If using overnight syncing, be aware of any campaigns you may be using with "send immediately" messages - you wouldn't want messages going out in the middle of the night to your contacts.