Why did he say this sentence? I don't understand his intention here.
He says "Hymn 253" as a way of being sarcastic to Phoebe. To understand the joke, I will go into a brief explanation of what happened during this scene. Chandler hears a knock at the door and he says "With my luck, that will be him." Phoebe then asks Chandler if it was him (she is talking about Ross). Chandler then snaps at Phoebe and says no, it's not him (Ross), it's a biblical hymn. A "hymn" is a song that is sung in churches. "Hymn 253" is a Christian gospel song that is named "His Eye Is On The Sparrow." "Him" and "hymn" are considered homophones: words that sound the same but are spelled differently. This is why Chandler made that joke; these two words sound the same but have different spellings and meanings.