"God is a Spirit, and those who worship Him must worship Him in the Spirit and in the truth", the Lord had said to the woman of Samaria who had quenched His thirst at the well.   (John 4: 23,24). 

In fact, He stressed that "true worshippers" will worship God in this manner - that is, "in the Spirit and in the truth".  It is therefore important for those of us who consider ourselves Christians, to see what the expression "in the Spirit" means.  So, let us allow the Apostle Paul to explain this to us:

"For the law of the Spirit of life in Christ Jesus has freed you from the law of sin and of death... for you are not in the flesh but in the Spirit, if the Spirit of God dwells within you.  If someone does not have the Spirit of Christ, then he is not His.... and if the Spirit does dwell within you, then the One Who raised Christ from the dead will also revive your mortal bodies through the indwelling Spirit within you.... for, those who are led by the Spirit of God, are sons of God...  (Romans 8: 2,9,11,14).

"This Spirit co-testifies together with our spirit, that we are children of God... Likewise the Spirit co-perceives our weakness; as to what we should pray for - the way we should - we do not know; but this Spirit beseeches on our behalf, with mute sighs.  The One Who examines the hearts knows what the thought of the Spirit is - that it beseeches to God on behalf of saints."  [Romans 8:16, 26,27] 

"...instead, be filled with the Spirit, saying (λαλούντες) to yourselves Psalms and hymns and spiritual odes, singing and chanting in your heart to the Lord, always thanking Him, for everything..."  [Ephesians 5:18-20]

According to these verses, one is "in the Spirit", when "the Spirit of God speaks within him" and directs him; when the Spirit speaks inside his heart with Psalms, with hymns, with spiritual songs; when it speaks to one's spirit with mute sighs.

One who is "in the Spirit" is any son of God who has been baptized with the Holy Spirit and is directed by the Holy Spirit.