If they were looking honestly and examining if their worldview gives them a solid foundation to define what is right and wrong, they would realize that there is no foundation to objectively define it. Moral values are reduced to personal opinion, and as such, lack objectivity, and cannot be binding. There are just molecules, stardust, matter, physics, that's it.
If there is no God, there are no objective moral values, since they are prescribed " ought to be's".
If there is no God, then moral values are just a matter of personal opinion, and as such, no objectively or universally valid at all.
If that is the case, unbelievers have no moral standard to judge anything as morally good or bad.
But they think they are on the morally high ground and by frequently arguing they are so, they want to highlight how righteous and honest they are, while constantly attempting to put the believer in God in a position where he is morally below them.
If we, as humans, are only physical systems (merely matter), we ought to stop trying to hold each other accountable for misbehaviour. In fact, there can be no misbehaviour if we are only physical brains and bodies; there can only be behaviour. Our actions have no moral content at all unless we truly have the freedom to choose and the ability to break the bondage of physical event causation.
Morals can't come from a natural contingent force, or nature since these are impersonal sources, and no morality can come from impersonal energy, matter, since there is no scientific rationale for energy/matter/nature providing a standard of behaviour and thinking that we ought to follow with true and right values, nor is there a means for it to translate it to humans, since impersonal objects/entities have no proven means of language to convey such moral standards. Only a personal being who is not contingent or temporal, maximally great, not just quasi maximally great, and eternal with language capability, and the means to convey his standards to humanity would be the only logical answer for a transcendent moral lawgiver that can actually have a conscious mind with language capability to formulate and articulate and express to humanity moral standards. A theistic God fits the bill nicely, who conveys his morality via conscience, the Word of God and the Holy Spirit.