here is what I responded with when I was faced with this objection.
"God himself is the standard for what is good. Good is whatever is in accordance with Gods Character. good is not good in of itself, it must have an ultimate standard.
Do you believe that good is simply good because it is good or do you believe that good is good because humans say so?
if the first then moral laws exist independent of man. if the second, then moral laws are nothing more than the opinions of man according to your worldview."
Don't just answer the objection, rather do what i did and turn it around on the one who is attacking the faith and try to steer the conversation toward the biblical Gospel. That we are sinners who only deserve hell, that god in his mercy and grace has decreed to only save some and died for those who he has set apart for salvation (the church aka the elect) in love. That the sins of the elect were imputed to christ on the cross and he bore the wrath of God against their sins by dying in their place and when one believes and trust in that gospel for salvation (which is how you know your elect, only the elect will understand, believe, and trust in the gospel) the righteousness of christ is imputed to that persons account immediately and he is declared justified by God. All that must be done by the individual to be saved is to believe and trust in the biblical gospel for salvation, not commitment to good works, good works is simply about being thankful and loving god for who he is and what he has done. never will a saved individual have to prove to anyone that he is good enough for god or good enough to be considered saved. its all grace.