Hebrews 9:15
Therefore he is the mediator of a new covenant, so that those who are called may receive the promised eternal inheritance, since a death has occurred that redeems them from the transgressions committed under the first covenant.

The Bible is clear that Jesus Christ is the only go-between, or mediator, that can bring together holy God and sinful humanity. [1 Timothy 2:5-6] Because of what Jesus accomplished by His sacrificial death, we now have redemption. 

In addition to redemption, Christ's work means we are promised an inheritance from God. "But when the goodness and loving kindness of God our Savior appeared, he saved us, not because of works done by us in righteousness, but according to his own mercy, by the washing of regeneration of renewal of the Holy Spirit, whom he poured out on us richly through Jesus Christ our Savior, so that being justified by his grace we might become heirs according to the hope of eternal life." [Titus 3:4-7}

So, then, we are heirs of God, co-heirs with Christ. [Galatians 4:6-7] And how can we know that God will keep His word and give us the promised inheritance? Primarily because He said so. God is faithful, and has proven himself to be faithful time and time again. Whenever the people of Israel turned from God, he did not destroy them, but remained true to his covenant with them. His word is his bond, and if God has said we will receive this inheritance, then we can believe with assurance that we will receive it. [2 Thessalonians 3:3; Hebrews 10:23]

But God, perhaps with our skeptical nature in mind, went a step further than his word by giving us his Spirit. We are guaranteed the full inheritance because the Spirit is a down payment, or deposit, that God has given us. [2 Corinthians 1:21-22; Ephesians 1:13-14]