You can be fired for not disclosing an arrest that is still on your record. It would be a better investment of your time and money to hire an attorney to see if you can seal (expunge) this from your record. Contact an experienced criminal defense attorney to investigate your criminal history and see what is eligible for expungement/sealing.
It also depends on what question was asked on the application. Employers are only allowed to ask about convictions, but can do so for any type of criminal conviction (misdemeanor and felony) with no time limit. So if you were not convicted you shouldn't have had to reveal the fact that you were charged. If the employer asked about arrests and you didn't reveal it, this is trickier, because they shouldn't have answered the question in the first place. If that's the case, you should contact the EEOC or seek an attorney if you are terminated.
The information is not, nor is it intended to be, specific legal advice. You should consult an attorney for individual advice regarding your own situation. Answering this question does not in any way constitute legal representation.