I understand your conundrum very well. I work in a large law firm, and I am certain that some of the lawyers engage in practices that fall outside the boundaries of morality. I believe that God places us in the midst of unbelievers to be witnesses and examples. Pope Benedict admonishes us to “emanate a fragrance of holiness” to everyone around us. Think of Joseph, serving the household of an Egyptian Pharaoh for so many years, and Daniel, a captive of the Babylonian King. God performed an important work through them, specifically because they were in those particular places. Think of the story of Naaman in 2 Kings chapter 5, who was healed of his leprosy by Elisha. Naaman was an army commander in a pagan land, and he explained that although he worshipped Elisha’s God (rather than the gods of his land), his master (the king) would lean on Naaman’s arm when he went into the pagan temple to pray. Naaman explained his difficulty to Elisha and asked for pardon.
"But may the Lord pardon your servant on one count: when my master goes into the house of Rimmon to worship there, leaning on my arm, and I bow down in the house of Rimmon, when I do bow down in the house of Rimmon, may the Lord pardon your servant on this one count.” (2 Ki 5:18-19).
I view my job as a vocation, and every morning I pray earnestly that I will be used as a silent example of holiness and faithfulness in the work that I do, humble though it may be. Perhaps God has placed you in your specific job to be an encouragement to a fellow worker or customer who is having doubts about their faith.