If we are “in Christ”  we have received a guarantee. God has already given us the “earnest” down payment of His Holy Spirit which guarantees that the rest of the inheritance will follow. The earnest (Greek ami-bow) is the initial down payment which guarantees both that the complete inheritance will be given and that the remainder will be of the same good quality as the down payment. The redemption of our body is clearly a part of our inheritance (Romans 8:23).  

Paul elaborated the function of the Holy Spirit as our earnest or guarantee in his letter to the Ephesians, where he wrote:  

“In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that holy Spirit of promise, which is the earnest of our inheritance until the redemption of the purchased possession, unto the praise of his glory .” (Ephesians 1:13, 14).  

In this passage, the Holy Spirit is pictured both as a seal and as an earnest, doubly emphasizing His work in giving us security and a guarantee. Barclay aptly says: “The Spirit of Christ is a first installment of the life to come.”