Silence is silence. It neither approves nor disapproves in itself. It is nothing.
However, we cannot honestly quote an author on something he never said.
If we cite an author as promoting and approving something, we better be able to show where and how he promotes or approves it. Otherwise, we have misrepresented the author.
Therefore, when an author has said nothing about a subject, we have no warrant to say that he promotes or approves of anything regarding that subject.
In other words, an author “authorizes” his views by what he says, not by his silence.
But if the author has not said anything about that subject, does that necessarily mean he disapproves?
The only way to know this would be if he breaks his silence on the matter. He might not necessarily disapprove, but how would we know? Yet once he breaks his silence, then this point is no longer at issue.
On the other hand, what if the author said something like this: “the only thing I’m approving of or promoting are matters that I have spoken about. I do not approve or promote anything else. Do not presume so.”
Then, when he is silent about a matter, what should we assume his feelings to be about it? We surely cannot assume that he approves of something he has not spoken about.
If, however, we are concerned about what the author in question really thinks, we will not presume to say he approves of something when he has said nothing about it. We will simply let it rest in silence.
"You shall not add to the word which I command you, nor take from it, that you may keep the commandments of the LORD your God which I command you" (Deuteronomy 4:2).
"Whatever I command you, be careful to observe it; you shall not add to it nor take away from it" (Deuteronomy 12:32).
"Do not add to His words, Lest He rebuke you, and you be found a liar" (Proverbs 30:6).
"I marvel that you are turning away so soon from Him who called you in the grace of Christ, to a different gospel, which is not another; but there are some who trouble you and want to pervert the gospel of Christ. But even if we, or an angel from heaven, preach any other gospel to you than what we have preached to you, let him be accursed. As we have said before, so now I say again, if anyone preaches any other gospel to you than what you have received, let him be accursed. For do I now persuade men, or God? Or do I seek to please men? For if I still pleased men, I would not be a bondservant of Christ" (Galatians 1:6-10).