'Guarantee' in the Bible

This is why the promise is by faith, so that it may be according to grace, to guarantee it to all the descendants—not only to those who are of the law but also to those who are of Abraham’s faith. He is the father of us all

And He will also confirm you to the end [keeping you strong and free of any accusation, so that you will be] blameless and beyond reproach in the day [of the return] of our Lord Jesus Christ.

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].

God has prepared us for this and has given us his Spirit as a guarantee.

who is the guarantee of our inheritance until God redeems his own possession for his praise and glory.

So Jesus has also become the guarantee of a better covenant.