Chicken-egg problem. They got their value system from their society, which got it from Christianity. That is where most religious people would stop looking. However, going further back, Christianity got its value system from the society that created it. All value systems are made by man.
First problem is a lack of understanding the word VALUE. VALUEs are derived from belief systems. For Christianity, our value system is based on foundations which were derived from Judaism and completed in the Person and teachings of Jesus Christ. For Christians this is no mere "man-made" value system. Christians believe from the testimony of the original Christians that Jesus Christ is the Son of God, therefore God Himself. Any values Christians hold to be true are therefore ordained by God and are ABSOLUTE on all creation. Jesus Christ, for Christians, is the Way, the Truth, and the Life.
On the other hand, any other values contrary to Christian values are meaningless to Christians.
What really blows peoples minds is the fact that Christians MUST replace the word VALUE with the word VIRTUE. Once that is done, then one can understand why the Christian can say...
Without God, where do they get their value system?
Virtues are moral absolutes that dictate human values.
From an objective view, the societal values of the original semitic tribes got put into the religion they created, Judaism. That progressed to Christianity as refined by a very wise man. These values have further changed through the years as the societies changed. Is there any Christian alive who would accept slavery as regulated by the Bible?
It is precisely the inability of a society to evolve beyond the commands in its holy book ("God's word") that is the reason for the Muslim violence in the world. Meanwhile, most Christians disassociate themselves from the likes of the Westboro Baptist crowd that's living with the values of a thousand years ago.
It has always amazed me atheist claim “No Godâ€. They take great effort to disprove this. The very fact they battle against something apart from themselves actually supports the fact that He is. Else who are they fighting?... perhaps themselves and the very Spirit trying to reach them.