Your cultural environment could influence you; your parents; reading; hearing from a close friend; through education; the media; entertainment, etc.
Would "actively taught at a very suggestible age" be among these reasons?
Most definitely and the Word of God says to train a person up in the faith when they are young so that they will not easily depart from it.
Deuteronomy 6:6-9 (NASB)
6 These words, which I am commanding you today, shall be on your heart. 7 You shall teach them diligently to your sons and shall talk of them when you sit in your house and when you walk by the way and when you lie down and when you rise up. 8 You shall bind them as a sign on your hand and they shall be as frontals on your forehead. 9 You shall write them on the doorposts of your house and on your gates.
and that from childhood you have known the sacred writings which are able to give you the wisdom that leads to salvation through faith which is in Christ Jesus.
This is often how american Christianity is transmitted.
For many, yes, and biblically so!
The null position that you're calling a pre-supposition is not taught this way, almost at all. The null position often comes from ideas like "Well, if I'm a Christian and I'm going to heaven, and my girlfriend is Jewish, she'd be going to hell, wait, how do I know I'm right and she's wrong?" lines of questioning. In other words, I find that religious positions and presuppositions are taught at a fundamental time in a person's development, while atheist / null positions are almost uniquely arrived at through independent thought. I don't think 'null' is presupposed in almost any case, it's almost always concluded, whereas religious positions, Christians or otherwise, are often the result of childhood inculcation.
While this can be the case, it is the foundational ideas that most influence us that we tend to cling to until something happens to test those foundations of our thinking. From kindergarten through college and university, the student is indoctrinated by a secular worldview and belief system. Naturally, when that is taught that is what is turned to in order to find answers. Thus, most continually build on the secular foundation. How well does it answer the ultimate questions of life? It fails miserably.
Leave the belief and build on another one or live with it knowing that it is not true. Sometimes people prefer the latter because they have invested so much into that belief system and it is a crutch for them.
I don't think many choose by feeling alone although I am not saying this can't be the case.
Are you saying you think many people who living with a belief knowing it's not true are doing so out of personal discomfort with the alternative?
That is not what I am saying in that sentence but yes, I do believe that is often the case, especially with Christianity. Christianity means you are not sovereign, you do not have the final say. You are morally accountable. That is scary for those who know they have done wrong. So, as Romans 1 says, they suppress the truth in unrighteousness.
What I am saying with that particular sentence is that some people who believe in God are willing to take Him exclusively on His word and do not need the other external verification. The eternal verification is often an apologetic or apology for the unbeliever, but it also strengthens our faith.