Can you give me a modern example of what blasphemy against the Holy Spirit is?

When you say it can't be forgive, is that living your whole life rejecting god or could it be converting to a religion just because you like it but coming back to God? Is it something the is long term, or can you commit it in a split second? If you do commit and you are true sorry will it count? I thought Jesus said that if you believe in Him then you are saved?