Question

In your book The First Existent, in the proof from morality (p. 407), you write as follows:

"The distinction between Mother Teresa and a hired murderer indicates that there must be some external standard involved in the moral judgment of a person or of an act. That standard is what determines which values are good or bad."
I did not understand why this has to be an external standard.

Why can’t the person himself determine what values are good or bad?

One could say that every person feels within himself what the good act is and what the bad act is. And the difference between Mother Teresa and a hired murderer is that Mother Teresa acted out of good will, according to what she thought was a good act. And the hired murderer acted even though he thought it was a bad act, out of a desire to receive payment or to satisfy his other interests.

If a person murders because he thinks it is a good act (for example, euthanasia), and acts out of good will and not out of other interests, then indeed it would be a moral act.
On page 409 you do indeed try to answer this question, but even there I did not understand the answer. This is what you write:
"Q: I assume that the atheist will raise here the possibility that the source of morality is in the heart or in the conscience… There is no need to arrive at God as the source of moral validity.

A: But this is a very strange claim. Our heart or conscience are elements built into us. I act according to them because that is how I am conditioned to act [Really?! So can we throw the book The Science of Freedom in the trash?] A person who is not equipped with such internal structures (in the extreme case: a psychopath) will not obey them."
And the question asks itself: after all, a basic assumption is that we have free choice, and we do not act one way or another because we are conditioned and compelled. And every healthy and normal person has desires for good and for its opposite.

Mother Teresa chose to act according to the good desire, and the hired murderer chose to act against the good desire.
Also regarding a psychopath, the accepted view today is that he understands that the act he did is defective and evil, and he chose to do the act despite that. Therefore he is held responsible for his actions. Even if he receives some mitigation in punishment, he is still punished severely.
Interim summary: one can say that every person has the good desire and the desires opposed to it. The moral person is the one who chose the good desire, and the immoral person is the one who chose the opposing desires.
Later on you argue: "When the source of validity for moral norms is within me, it is by definition subjective and non-binding. In order for it to have binding validity, it must have an external source."

And I do not understand why a subjective internal source cannot be binding. And why can an external source be binding?

Why does it matter whether the source is external or internal when it comes to creating obligation or meaning?
In addition, you argued that if the source is internal and subjective, then it varies from person to person, and therefore it does not bind all human beings.

So what? Why does morality need to bind all human beings? Why is it not enough that it bind everyone who experiences it within himself?

(And I claim that morality is subjective but shared by all human beings. Every normal human being sees harming another person as something bad and improper.)

Summary: one can say that morality is determined by each person’s subjective feeling, and a person who acts according to this feeling is moral, while a person who acts against it is not moral. Validity, obligation, and meaning are also part of the subjective feeling of human beings. And if so, this does not lead to a proof of the existence of God.
 
Thank you very much for your time.