When A believes that the proposition p is true, we may say
A subscribes to the proposition that ...
Which verbs may replace the italicized expression without altering the meaning of the sentence?
Answers should be either suitable to academic prose or archaic.
One may espouse or embrace a proposition, particularly if it is a deeply-held belief.
If the person actively supports the proposition, in addition to simply believing it is true, you could also say that they champion, endorse, or advocate it.
"Believes"!
There is also "accepts", and possibly in some contexts "adopts".
A adheres to the proposition that ...
The usage especially fits the general idea supported by subscribing to an idea: one is supporting or attesting to that idea. In the Merriam-Webster dictionary, "Adhere" refers to supporting or assenting to an idea (meaning 2):
1 : to hold fast or stick by or as if by gluing, suction, grasping, or fusing
The stamp failed to adhere to the envelope.
2 : to give support or maintain loyalty
adhere to traditional values
3 : to bind oneself to observance
adhere to the rules
concurs with, a little more formal than agrees with.
In British English, “agree” is sometimes used with a direct object, e.g., “agree the treaty.”
