2Th_2:13 But we are bound to give thanks alway to God for you, brethren beloved of the Lord, because God hath from the beginning chosen you to salvation through sanctification of the Spirit and belief of the truth:
I'll offer the simple answer to this problem. Believe and belief are words with definitions, and those definitions do not include knowledge.
The above scripture has the word "belief" in it, and the definition of this word (according to Strongs) is:
G4102
πίστις
pistis
pis'-tis
From G3982; persuasion, that is, credence; moral conviction (of religious truth, or the truthfulness of God or a religious teacher), especially reliance upon Christ for salvation; abstractly constancy in such profession; by extension the system of religious (Gospel) truth itself: - assurance, belief, believe, faith, fidelity.
I see these words for believe:
G4100
πιστεύω
pisteuō
pist-yoo'-o
From G4102; to have faith (in, upon, or with respect to, a person or thing), that is, credit; by implication to entrust (especially one’s spiritual well being to Christ): - believe (-r), commit (to trust), put in trust with.
G4103
πιστός
pistos
pis-tos'
From G3982; objectively trustworthy; subjectively trustful: - believe (-ing, -r), faithful (-ly), sure, true.
These words do not indicate knowledge having anything to do with belief or believe, but I do understand that to believe is a step towards knowledge of the Truth (knowing Jesus/Truth/Good).
To believe not is defined as:
G569
ἀπιστέω
apisteō
ap-is-teh'-o
From G571; to be unbelieving, that is, (transitively) disbelieve, or (by implication) disobey: - believe not.
G571
ἄπιστος
apistos
ap'-is-tos
From G1 (as a negative particle) and G4103; (actively) disbelieving, that is, without Christian faith (specifically a heathen); (passively) untrustworthy (person), or incredible (thing): - that believeth not, faithless, incredible thing, infidel, unbeliever (-ing).
This brings me to the simple answer to all of this, and this is the shortest answer I can come up with concerning this issue of so many saying so many different things, and yet all confess to be Christian:
1Co 1:10 Now I beseech you, brethren, by the name of our Lord Jesus Christ, that ye all speak the same thing, and that there be no divisions among you; but that ye be perfectly joined together in the same mind and in the same judgment.
1Co 1:11 For it hath been declared unto me of you, my brethren, by them which are of the house of Chloe, that there are contentions among you.
So how do we all speak the same thing, easy, and it has nothing to do with what we believe.
Joh 3:11 Verily, verily, I say unto thee, We speak that we do know, and testify that we have seen; and ye receive not our witness.
Only speak what you know, and keep your beliefs to yourself, until you know one way or another, and that is the answer.
Joh 8:31 Then said Jesus to those Jews which believed on him, If ye continue in my word, then are ye my disciples indeed;
Joh 8:32 And ye shall know the truth, and the truth shall make you free.
It's a great topic, and there is no doubt that we ought to speak what we know, not what we believe.
2Ti 1:12 For the which cause I also suffer these things: nevertheless I am not ashamed: for I know whom I have believed, and am persuaded that he is able to keep that which I have committed unto him against that day.