The New Covenant is sealed by the Holy Spirit. In Ephesians 1:13-14, Paul says, "And you also were included in Christ when you heard the word of truth, the gospel of your salvation. Having 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."This tells us that we are in Christ (i.e. saved) from when we hear the word of truth and believe it. It further tells us that the seal marking us in Christ is the Holy Spirit within us. The Holy Spirit described as the deposit, or down payment, that God makes ensuring us that we are going to be with him. Because we have the Holy Spirit within us, giving us a direct connection to God, we can know that we have salvation and that his covenant is secure.AnswerThe New Covenant was sealed with the blood of Christ "This cup which is poured out for you is the new covenant in my blood." (Luke Ch.22)