An authentic signature makes things legal.  It is said that we should be careful what we sign and read all the fine print first.  Signatures are binding.

Teachers will send home report cards to be signed by a parent.  This signature guarantees they are aware of the progress of their child.

An autograph from a famous person on a photo, football or even a slip of paper can be worth a lot depending on the accomplishments of that celebrity.

A signature on a document makes everything legal and guarantees that what is signed will actually happen.

The most valuable signature is the one God places on us.  His Holy Spirit is written on our hearts.  His signature guarantees us an eternity with Him filled with happiness, peace and joy.  His signature guarantees we have been bought and paid for in full by Jesus Christ.  His signature guarantees He will take care of us in all things.  We are official because our hearts are a signed document of love.

2 Corinthians 1:21-22

It is God who enables us, along with you, to stand firm for Christ. He has commissioned us, and He has identified us as His own by placing the Holy Spirit in our hearts as the first installment that guarantees everything He has promised us.