**They may have a conscience, but the moral law they follow is to do whatever seems right to themselves.**
Not my experience. Many agnostics/atheists I know had religious parents/traditions but did not attend services regularly. IMHO public education and peers successfully inserted questions of faith and confusion in their minds. Nevertheless, decent people do not discard morals while they are seeking answers to greater questions of faith and theism. To suggest they do is self-righteous - and sinful.
I will overlook the provocative challenge of self-righteousness and sinfulness. To both charges I plead guilty.
But I wonder what might be the definition of a "decent" person. From what I know, all have sinned and fallen short of the glory of God. So decency, apart from God, tends to be a general acceptance of cultural norms because it seems reasonable.
But cultural norms can change at the drop of a hat. If the individual does not change their values when the culture changes it is either because those values are timeless, received from God and accepted by faith, or they remain convinced of their own subjective capacity for determining good an evil. It was that belief in ones own capacity for determining good and evil that led to the Fall in the first place.
I am not saying that there is no role for reason in seeking understanding of good and evil. I welcome the atheist/agnostic to enter into the discussion. But in the end, we either have morality stamped in our hearts by our Creator or we have a philosophical system to which we give intellectual assent. Sadly, the latter, a stance to which we all fall prey at times, is insufficient, because it still reduces to "whatever seems right to me." That, my friend, is the purest definition of "self"-righteousness.