If anyone does not have the Spirit of Christ, he is not His.

If the Spirit of Him who raised Jesus from the dead dwells in you, He who raised Christ from the dead will also give life to your mortal bodies through His Spirit who dwells in you.
Romans 8:9, 11


Jesus told His disciples, "I will pray the Father, and He will give you another Helper, that He may abide with you forever -- the Spirit of truth, whom the world cannot receive, because it neither sees Him nor knows Him; but you know Him, for He dwells with you and will be in you."
John 14:16-17


Those who have trusted Jesus Christ for salvation are His, and every person who is His must also have the Spirit of God.