What does that statement say about your ability to understand what the Bible really teaches?
A great deal.
There are many good Christians who believe in the deity of Christ and don't believe the Trinity doctrine.
What?
Are you making things up as you go now or what?
What do you consider "many", two, three maybe ten? I bet you can't prove the existence of even that many and even if you could, I'm not arguing on the basis of popularity (or the lack thereof) anyway. The number of people who believe something has nothing to do with whether it is true or not.
Further, and more importantly, if you consider yourself to be among these "many Christians" you have already conceded the debate and don't know it because if there can two members of the Godhead, there can be three. No objection you can think of to refute the later won't also refute the former unless you want to take on the task of arguing that the Holy Spirit is not God.
The reason you can't persuade them that the Bible teaches the Trinity is because the Bible doesn't teach the Trinity, the church teaches it.
You can't come up with any rational argument to try to persuade them because the Trinity is not a rational doctrine, it is a doctrine of "faith".
Actually, the version of the Trinity doctrine that the church most often teaches is quite different from that which the bible teaches and cannot be defended biblically or rationally because it is intentionally irrational and is always defended as "a doctrine of "faith", as you suggest. They teach that God is somehow both plural and singular - period. In other words, they teach that the adjectives "plural" and "singular" describe God
in the same context. I do not make any attempt to defend that doctrine.
You bring up rationality and I feel it necessary to define that term. There is one fundamental building block of all knowledge, understanding and meaningful discourse. We call it the law of identity. What is, is. A is A. Everything you know, everything you've ever understood and every syllable of meaningful communication you've ever experienced is all entirely based on that single truth and it's two corollaries, the law of contradiction which states that any two truth claims that contradict each other cannot both be true
in the same context (i.e. at the same time and in the same way) and the law of excluded middle which states that a truth claim is either true or it is false given its specific context. These laws were not invented or created, they were discovered. They are the laws that reality follows and as such to be rational simply means to conform one's mind to reality.
The doctrine of the Trinity as taught most commonly by the church stands in direct conflict with the law of contradiction because I stated before they teach that God is both singular and plural in the same context. It is essentially an overstatement of what the bible teaches.
So what does the bible teach?
It teaches that there is one God and only one God.
It teaches that the Father is that God.
It teaches that the Son, who became a man whom we call Jesus Christ and who died and rose from the dead is that God.
It teaches that the Holy Spirit is that God.
It teaches that the Son is not the Father nor is He the Spirit and neither is the Spirit the Father.
In regards to specifics, that's pretty much all it teaches. The bible does indicate right from the very first sentence of the bible that there is a plurality in God but no details are given about this beyond what I've just listed.
So, how is that any different than what the rest of the church teaches?
Well, as far as I've gone to this point, it isn't. It's the conclusions they draw that are where they go too far. They presume that because the above listed biblical facts are undeniable that, therefore, God is both singular and plural and they intentionally state it as a contradiction that they are willing to accept as true on "faith". The problem for them is that the bible doesn't teach it as a contradiction and does not ask us to accept it as such. On the contrary, I am content to believe the biblical facts and leave it at that. There is clearly some way in which a plurality exists within the SINGULAR God.
The Father, Son and Spirit are distinct from one another in one sense but are also One God - and here's the important bit -
IN SOME OTHER SENSE. The details of how this works are not explained to us but the correct response to this lack of information is not to throw reason in the sewer and accept a contradiction as truth. To do that would render any wild-eyed insanity that any cult leader wanted to teach utterly unfalsifiable. It is not faith to accept stupidity as truth. It is, however, faith when one says to God that I accept your word and will await further understanding when you choose to provide it.
Clete