[concinds]

The argument isn't "you can only be moral if you're religious", it's: "modern Western moral attitudes, even explicitly atheistic ones, are strongly influenced by historical religious ideas, and therefore the idea that religion is inherently immoral is incoherent; and contempt for religion is unjustified." Seems reasonable to me.

[hackyhacky]

Your argument ignores cause and effect.

Religion was created by man, not by god. Religious ideas, including religious morality, are the results, not the causes, of the prevailing cultural attitudes of the societies that created them.

In the modern West, such as the US, "religious morality" is mostly synonymous with the morality of the ancient societies in which those religions originated, which is why religion is so out of step with modern sensibilities.

[swat535]

Thomas Aquinas saw a connection between the metaphysical existence of God and the values of morality in the sense that they are both grounded within His existence.

In other words, Aquinas saw God as the basis for and to all we experience. From the theistic perspective, my question is if moral subjectivism is true, it seems to be self defeating as the concept of truth is fundamentally underwhelmed.

There are plenty of atheistic and some theistic philosophers who see morality as independent of God. I wouldn't necessarily agree with them, but it's just a thought