Perhaps the reason his answer didn't make any sense to you is because it doesn't make any sense! In other words, his hostility toward God or Jesus isn't based on logic or reason, but simply comes from his emotions.
He would probably deny this, of course, perhaps thinking he's being very logical when he reacts this way. But in reality, he isn't. Instead, he has a deep-seated hatred toward God, and whenever anyone even mentions God or says something favorable about Jesus, he lashes out in anger and hostility. He even refuses to consider the possibility that he might be wrong, and his mind is completely closed. As the Bible says, "Fools find no pleasure in understanding" (Proverbs 18:2).
Why is your colleague like this? Only he knows the answer (if he's ever really thought about it). He may, for example, be reacting against his parents' faith, or perhaps he likes to think he's smarter than people who believe in God. But the real answer is deeper than this: He simply wants to run his own life. He doesn't want anyone telling him what to do or how to live -- especially God.
Pray for this person; only God can break through the hard shell he's built around his mind and heart, and convince him of his need of Christ. And pray too that your life will be a living testimony to Christ's love and joy. Remember Jesus' words: "You are the light of the world" (Matthew 5:14).