13 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, 14 who is a deposit guaranteeing our inheritance until the redemption of those who are God’s possession—to the praise of his glory.
“The Holy Spirit is God’s seal that we belong to him and his deposit guaranteeing that he will do what he has promised. The Holy Spirit is like a down payment, a deposit, a validating signature on the contract. The presence of the Holy Spirit in us demonstrates the genuineness of our faith, proves that we are God’s children, and secures eternal life for us. His power works in us to transform us now, and what we experience now is a taste of the total change we will experience in eternity.” (Life Application Study Bible 1976)
Life Application Study Bible. Zondervan, 2011. p. 1976.