My iPhone 6s Plus was picked. Quick question

Hi there, my iPhone 6s Plus was picked from me last night and i had Find iPhone turned on and stuff like that, my SIM has a code in order for it to function and my phone is also locked. Immediately after my phone was picked someone turned the phone off and i know this because i immediately called the phone using someone else phone and it went straight to voice mail and when i checked Find My iPhone is said that the phone is offline. I put my phone on Lost Mode and left it just like that however what is going to happen is that the person who stole my phone will not have any use since they turned it off which cause the phone to then require password to unlock and also the SIM needs to be unlocked so i would like to find out if i erase my iPhone content and settings will it still be closed to the person who stole the phone or is there any slight chance of the person who picked my pocket to unlock the phone through High Technology software?