"By this we know that we abide in Him and Him in us, because He has given us His Spirit." 1 John 4:13 - The Holy Spirit is a sign of truly being in Christ.
1 John 3:24 says basically the same thing... " The one who keeps His commandments abides in Him, and He in him. We know by this that He abides in us, by the Spirit whom He has given us."
Romans 8:16 has more to say on that subject: "The Spirit Himself testifies with our spirit that we are children of God,"
"In Him, you also, after listening to the message of truth, the gospel of your salvation - having also believed, you were sealed in Him with the Holy Spirit of promise, who is given as a pledge of our inheritance, with a view to the redemption of God's own possession to the praise of His glory." Ephesians 1:13 & 14
"But if the Spirit of Him who raised Jesus from the dead dwells in you, He who raised Christ Jesus from the dead will also give life to your mortal bodies through His Spirit who dwells in you." Romans 8:11
"In the same way the Spirit also helps our weakness; for we do not know how to pray as we should, but the Spirit Himself intercedes for us with groaning too deep for words; and He who searches the hearts knows what the mind of the Spirit is, because He intercedes for the saints according to the will of God." Romans 8:26 & 27 Isn't that super cool!??
Do you have the Holy Spirit in your life? Acts 2:38 has something to say about receiving the gift of the Holy Spirit. Check it out! :-)