As a former employee of the Home Depot, I have to say that this guy has no one to blame but himself. The dress code, to which you must implicitly agree to follow when hired, is extremely clear on the issue of augments to your uniform. You cannot wear anything under or on your orange apron which is either not already approved by the Home Depot or cleared with the store manager.
My experience (at my store at least, although the "culture" of the company is pretty homogeneous across the country), is that you are better off not wearing any kind of button or ribbon or augment at all, as the majority of them are considered inappropriate. One employee was asked to remove her Chicago Bears button during their Super Bowl run some years ago, so the policy does not necessarily discriminate against religious icons; more accurately it would be said to discriminate against "fun", but I digress.
A couple of you have danced around what I believe to be the core issue here. The culture of the company facilitates the formation of cliques within the store, some more favored by management than others. It is very possible that some of the more influential employees at this store simply did not like this person, whether because of his open religious beliefs, his personality, his work ethic, whatever. Due to that dislike, they most likely used their pull with management and reported him.
Why wait a year to ask him to remove it? Since he was not directly harming anyone, nor was he openly preaching his beliefs to others, management probably looked the other way, especially if he was a competent employee. Until he started bringing the bible to work with him that is. Unfortunately that was probably the straw that broke the camel's back, further offending those in the store that did not like him, and he was then reported, asked to remove the button, then predictably fired when he refused.
This incident is most likely the outcome of a string of incidences going back to whenever he was hired, coupled with the fact that he was probably not favored by management, but allowed leeway because he was good at his job. All of this is obviously speculation on my part, but I would be very surprised if it weren't true.
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Stop looking at me like that.