Ephesians 1:13-14 and a comment on

In him you also, when you heard the word of truth, the gospel of your salvation, and believed in him, were sealed with the promised Holy Spirit, who is the guarantee of our inheritance until we acquire possession of it, to the praise of his glory.

The Holy Spirit--God Himself--is the guarantee of our inheritance until we acquire possession of it; no better guarantee [guarantor] than God Himself.