In my opinion there are two problems with the above question:

First, a condition has no meaning when it comes to “belief.” How can someone believe conditionally? Either you believe (to the required degree), or you don’t.
Second, the condition “that He is not the God whom so-and-so worships, and that keeping His commandments is not in the way that so-and-so keeps them” is not defined.
I assume that generally you know what so-and-so believes and what he observes. So in what way, and to what extent, do you want to be different from him? That is really not clear. Could you elaborate?

Please don’t involve my grandfather in your affairs.

Yanky, young boy, keep your fingers out of matters that are not yours.

Anything you can do is possible.

Rabbi, what do you think about the claim I raised here, that one cannot make a condition regarding belief?

That is connected to the discussions about conditions regarding commandments. I’m not sure you’re right. The whole meaning of a condition is decisiveness of intent subject to the condition. The same question could be asked regarding acquisition as well: either there is decisiveness of intent or there isn’t. A person who is in doubt can perhaps make a condition in his service of God. I wouldn’t rule it out.
As for how well-defined the condition is, that’s an unimportant technical question. So we’ll define the condition better.

As for conditions, it seems to me there’s a difference between a condition that takes effect now, which you can make, and something retroactive, no? I seem to remember that Rabbi Shlomo Zalman Auerbach, in the laws of tefillin in Halikhot Shlomo, raises something like this regarding a heretic. See there.

Rabbi,

In response to a question here on the site, you yourself defined it this way: “Belief is knowledge. ‘I believe’ means ‘I know.’ These are synonymous words. Belief comes from the root truth.”
I understand that one can make a condition regarding a legal effect, such as an acquisition or a commandment.

Here, according to the questioner, he is making a condition not only on the commandment but on the belief as well, which according to your definition means on the knowledge. This is his wording: “I take it upon myself to believe in God… on condition that He is not the God whom so-and-so worships.”

In other words: “I know that God exists, on condition that Chaim K. does not worship Him.” So do you know or not know?

Could you explain this to me?

Rabbi,

In response to a question here on the site, you yourself defined it this way: “Belief is knowledge. ‘I believe’ means ‘I know.’ These are synonymous words. Belief comes from the root truth.”
I understand that one can make a condition regarding a legal effect, such as an acquisition or a commandment.

Here, according to the questioner, he is making a condition not only on the commandment but on the belief as well, which according to your definition means on the knowledge. This is his wording: “I take it upon myself to believe in God… on condition that He is not the God whom so-and-so worships.”

In other words: “I know that God exists, on condition that Chaim K. does not worship Him.” So do you know or not know?

Could you explain this to me?

You translated it incorrectly: If God exists, then I worship Him. On the possibility that I have knowledge, I worship Him.
Notice that every condition suspends a necessary requirement. As I brought regarding acquisition: in principle, decisiveness of intent is required, and nevertheless, if I do not have full decisiveness of intent, I transfer ownership on the possibility that there will be decisiveness of intent.

A Guy Hoping,
I didn’t understand. A condition “from now” is a retroactive condition (as opposed to a condition of “if”). I don’t understand why there would be any difference between the two conditions for our purposes.