John 14:15-17 “If you love Me, keep My commandments. And 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.
Romans 8:8-9 “So then, those who are in the flesh cannot please God. But you are not in the flesh but in the Spirit, if indeed the Spirit of God dwells in you. Now if anyone does not have the Spirit of Christ, he is not His.”
These verses assure each of us that we are actually living “in the Spirit,” because He has come to “dwell in you.” One commentary states that all of this has come about “not by works of righteousness which we have done, but according to His mercy, by the washing of regeneration, and renewing of the Holy Spirit”. This glorious new birth, with sins forgiven and eternal life, is accomplished by the Holy Spirit in response to our faith in Christ as Savior and Lord.
But also note that “if any man have not the Spirit of Christ, he is none of His.” It is absolutely vital that we have the Holy Spirit, “for as many as are led by the Spirit of God, they are the sons of God” - Romans 8:14. The question is, just how do we know that we have the Holy Spirit? The answer is, because His Word says so! “He that hears my word, and believes on Him that sent me,” said Jesus, “has everlasting life” – John 2:24. Furthermore, we have the testimony of internal peace and assurance. “The Spirit [Himself] bears witness with our spirit, that we are the children of God” – Romans 8:16.
Finally, the indwelling Spirit will increasingly be growing His eternal fruit in our lives—the nine fold fruit of “love, joy, peace, longsuffering, gentleness, goodness, faith, meekness, temperance” - Galatians 5:22-23, and this will testify to others also that we do indeed have the Holy Spirit.