Very easy if you know how. Getting past the keychain intact is the hard part. Bottom line a.) your passwords in the Keychain are probably secure since they would have blow away the keychain without the password. Your unencrypted data is vulnerable. Hopefully you did timely Time Machine or Carbon Copy Cloner backups and backed up your Apple I.D.- controlled data to iCloud. It probably already too late. If you have Find my Mac activated you can take another iDevice or log into on anotherputer and erase the unit and restore the data on a new Mac from your backups. Most home insurance policies cover theft. Good luck.