Ephesians 1:13

Ephesians 1:13 In whom you also, having heard the word of the truth, the gospel of your salvation, in Him also believing, you were sealed with the Holy Spirit of the promise,

To believe is our responsibility, and to seal is His. At the same time we believed in Christ, we were sealed with the Holy Spirit. For us to be sealed is to have the Spirit Himself within us as a divine mark. God has put the Holy Spirit into us as a divine mark to prove, testify, and declare that we are His inheritance. How can we know that we are God's inheritance? It is because the Bible tells us that God has confirmed our believing in Him by sealing us with His Spirit. Also, the sealing Spirit Himself deep within confirms that we have been marked out and gives us the likeness and appearance of God. This divine appearance and likeness is the mark, the seal, that confirms that we belong to God as His chidlren and His inheritance. 

- "the Divine Spirit with the Human Spirit in the Epsitles" P68