'Fulfillment' in the Bible

it is He who has also put His seal on us [that is, He has appropriated us and certified us as His] and has given us the [Holy] Spirit in our hearts as a pledge [like a security deposit to guarantee the fulfillment of His promise of eternal life].

Now He who has made us and prepared us for this very purpose is God, who gave us the [Holy] Spirit as a pledge [a guarantee, a down payment on the fulfillment of His promise].