Acts 1:8, 2:4
..."But you shall receive power, after the Holy Ghost is come upon you:: and ye shall be witnesses unto me...."
...And they were all filled with the Holy Ghost and began to speak with other tongues as the Spirit gave them utterance.
Being filled with the Holy Spirit is no more mystical than being born again! Both are a gift from God and both are received by faith in Christ Jesus. The 120 that experienced the filling of the Holy Spirit on the day of Pentecost (Act 2:1) were waiting in faith on the promise of the Father (Luke :24-49). The Holy Spirit was and is that promise.
Spiritual endowment is given to the believers so that they may be efficient and effective witnesses to the finished work of the cross. The Spirit working through the believers as they proclaim the Gospel, convicts of sin and convinces the sinner of the need to turn away from sin and turn toward God.
He illuminates the spoken truth concerning Jesus and gives hope for every situation to all who believes in the Son of God. The disciple's ministry was to begin in their local region, spread to surrounding areas, and then go to the remainder of the earth, making disciples as they went. This is still the commission of every believer. Christ promised that those who believed could operate in power and authority; and He would work with them confirming the Word with signs following
John 16:7 Nevertheless I tell you the truth; It is expedient for you that I go away: for if I go not away, the Comforter will not come unto you; but if I depart, I will send him unto you.