I don’t know exactly why this person gets so upset whenever God is mentioned—but have you ever thought about asking him? You might not get a calm answer, but it would be interesting to see what he says.
I have found that people like this often have “reasons” for their anger at religion and God. For example, your colleague may have had a bad experience with a church when he was growing up, and as a result he claims that all Christians are hypocrites and their faith is false. Or his parents may have been too strict or unwise in the way they tried to teach him about God, and he’s still rebelling against them and their beliefs.
But whatever his stated reason, the real reason why he gets so upset is probably far deeper. The real reason may be that he wants to run his own life, and he doesn’t want God to interfere. Or he may even be filled with pride, convinced he can get through life in his own strength. To him, turning to God may be a sign of weakness, and he doesn’t want anything to do with that. The Bible says, “In his pride the wicked does not seek him (God); in all his thoughts there is no room for God” (Psalm 10:4).
Pray for this person; only God can break through the hard shell he has built around his heart. Ask God too to help you be a witness to him of Christ’s love, both by what you say and the way you live. He may hate God, but God still loves him.