There are 3 different Greek prepositions that describe our relationship with the Holy Spirit – ‘para’, ‘en’ and ‘epi.’
1) PARA (WITH YOU) - The Holy Spirit is with a person, even prior to conversion. He convicts them of sin, righteousness and judgement to come (John 16:7-9) - and convinces them that Jesus is the Saviour.
2) EN (IN YOU) - When anyone calls on God to be saved, the Holy Spirit then comes INTO that person’s life.
1 Cor 6:19-20 Do you not know that your body is a temple of the Holy Spirit, who is in (EN) you.
3) EPI (UPON YOU) - But there is a third relationship that a believer can have which is separate and distinct from the first two. In Acts 1:8 Jesus promised “… but you will receive power when the Holy Spirit has come upon (EPI) you” (NASB)
Pentecostals maintain that the EPI experience (baptism in the Holy Spirit) is a unique experience, which is distinct from the EN experience (salvation).