“And you also were included in Christ when you heard the message of truth, the gospel of your salvation. When you believed, you were marked in him with a seal, the promised Holy Spirit, who is a deposit guaranteeing our inheritance until the redemption of those who are God’s possession—to the praise of his glory.” Ephesians 1:13-14 NIV

It is a shame that in our world an inheritance is not guaranteed. The family fortune may not be handed down to you. A precious heirloom may be given to someone else. Your right as a child to inherent is not always guaranteed in this life. But as a child of God bought by the blood of Jesus Christ you are guaranteed an inheritance.
At the moment we receive Christ as our savior the Holy Spirit takes up residence within us. Not only is the Holy Spirit dwelling in us to guide us through this life, but He is the assurance of our salvation and eternal inheritance. The apostle Paul says we ‘were marked in Him with a seal.’ It is a seal of authenticity, authority, security, and ownership. We belong to Christ. Jesus purchased us and has given us His Holy Spirit as a pledge (deposit) guaranteeing our future inheritance.
There are not many guarantees in life that are 100% guaranteed, such as a product you may have purchased. The way you have treated or mistreated the item will determine if the guarantee is honored. Our salvation is guaranteed. There is nothing we can do to lose our salvation and our eternal inheritance. It does not depend on us as it did not depend on us when we trusted in Christ. It was the work of the Holy Spirit who led us and we are now sealed guaranteeing of our inheritance. Rejoice in that today!