In him you also, when you heard the word of truth, the gospel of your salvation, and believed in him, were sealed with the promised Holy Spirit,  who is the guarantee of our inheritance until we acquire possession of it, to the praise of his glory. (ESV)
So many people struggle with uncertainty and fear in regards to their relationship with God. How secure is our relationship? Does God hold on to us no matter what? Can I lose my salvation? How can I know if I belong to Jesus? In these two verses we find solid answers and real hope. Let's look at our responsibility and the Lord's responsibility in this relationship.
In verse 13 we discover two significant elements of our salvation that fall on us. We must hear the Gospel, and we must believe the Gospel. The Gospel is the good news that Jesus Christ gave his life in payment of our sin, and that his resurrection provided for our eternal life. We must hear and believe. There are many people who attend church, do "good works," and yet have never believed in the truth of the Gospel of Jesus Christ. If you have not heard and believed the truth in Jesus Christ, you have good reason to feel insecure and fearful in your relationship to God. You are still lost. But, if you have heard and believed the Gospel, you belong to the Lord.
 But to all who did receive him, who believed in his name, he gave the right to become children of God, (ESV)
In verse 14 of our passage in Ephesians 1, we discover God's responsibility in this relationship. If we have heard the Gospel and believed in Jesus Christ, then we are sealed with the Holy Spirit. That seal is a mark of identification. We belong to Him. His Spirit dwells within us and we are under His authority. If people question our authenticity, they need to realize that our authenticity as believers in Christ comes from whose Spirit dwells within us. And while our outward works are incomplete, and we still struggle with sin, our relationship with God is genuine because he dwells within us. Lastly, how about our security? How secure is our relationship? The Holy Spirit, "is the guarantee of our inheritance until we acquire possession of it, to the praise of his glory." Inheritance . . . guaranteed!
Are you filled with fear and uncertainty in your relationship with God? There is no need to be. Receive and Believe the truth of the Gospel. Rest in the fact that those who have received and believed in Him are forever his, sealed by the very Spirit of God. You are secure. Your inheritance is guaranteed.