Formal visitations to funeral homes provide the opportunity for friends and family members of the deceased and the deceased's family to offer their feelings of sorrow and sympathy. Sending flowers to a funeral home along with a card or note of sympathy is a common practice if the family of the deceased has not made any requests to the contrary. Messages in cards that accompany flowers should be short and express your feelings without being stern, verbose or overly personal.
If you're inclined to do so, the message in your card can be a verse from the Bible. Appropriate verses are ones that comfort and soothe, like Psalms 91:1-2 or Philippians 1:20-23. The verses you write should be appropriate to the occasion. You can also write such things as "May God watch over you in your time of need". If the family of the deceased is not overtly religious, then avoid this type of message.
If you're inclined toward sympathy, the message on your card can express that. You can write comments like "May these flowers help to express our heartfelt sympathy" or "Please accept our deepest sympathy for your loss." It is appropriate to write a short, personal note about your relationship to the deceased or to mention what you will miss most about him or her. Write clearly and with a sympathetic tone.
Many authors have quotes that are appropriate for funeral flower cards. You can use C.S. Lewis's "Has this world been so kind to you that you should leave with regret? There are better things ahead than any we leave behind" or Hans Christian Andersen's "A human life is a story told by God." You could use Thomas Campbell's "To live in hearts we leave behind is not to die" or David Seltzer's "For some moments in life there are no words." Regardless of what you use, make sure you understand the broader connotations of the quote, especially if it is an excerpt from a larger piece, and that it is appropriate to the occasion.