For a Golden Anniversary, consider "You are an inspiration to anyone who is looking for an example of commitment" or "I wish you both much happiness and joy as you celebrate these 50 wonderful years together." A simple option is "Heartfelt congratulations on 50 incredible years together!"
For religious Golden Anniversary wishes, consider "I don't know how anyone could stay together as long as you have, unless God helped you do it!", "Congratulations on your golden wedding anniversary. May God shower His love and blessings on you for many years to come", or "I think you are a good example of the kind of commitment God intended in marriages."
For funny messages, consider "They say that scientists have found that the longer people are married, the more they start to look the same. After 50 years, you will need name tags sewn in your underwear," "A golden wedding anniversary is the celebration of trust, love, partnership, tolerance, and tenacity. The order may change for any given year," "Happy 50th anniversary to a couple of people who maybe shouldn't have gotten married, but somehow made it work all these years" or "Love is the thing that allows a woman to sing while she cleans up the floor after her husband has walked across it in his muddy boots."