I will pray for this lost soul. The gospel of Love often divides, but your friend is not being truthful with you or himself. If he really believed that there was no God, then you talking about Him would be no more offensive to him that talking about Santa Claus. It is evident that your sharing struck a nerve- one does not react that passionately about something they do not believe in. Like most "atheists " he lives in denial: His reaction is that of one who hates God, not of one who does not believe in Him. It is easier to reach someone (in time) who lives in this denial then it is to reach a blind man who truly does not believe- Keep praying.
One more thing I would like to add- a friend once told me this and it stuck with me : it is sometimes necessary to build a bridge of friendship that will sustain the weight of truth".
Psa 62:5 My soul, wait thou only upon God; for my expectation is from him.