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, (Ephesians 1:13)
God has placed a seal on those who have believed in Him. This seal is the Holy Spirit which was promised and is a sign that we are now God’s property. What belongs to God is God’s and cannot be taken by another. God ferociously guards His property and will allow nothing to separate us from Him.