We already have the Holy Spirit when we accepted Jesus. Why do we need to be filled/baptized again?

The Bible unequivocally declares, "Be filled with the Spirit" (Eph 5:18). An analysis of the Greek verb translated "be filled" shows that it is in the present tense, indicating that this blessing is one that we may experience and enjoy now. The fact that the verb is a command (imperative mood) does not leave the responsive disciple an option in the matter. However, since the verb is in the passive voice, it is clear that being filled with the Spirit is not something the Christian achieves through his own efforts, but is something that is done for him and to which he submits. Hence, the Scriptures depict a theocentric view of the Holy Spirit's filling, in which the Higher reaches down to father up the lower into ultimate communion. Clarity on this point dismisses the criticism or misunderstanding of some who seem to see this experience as something merely conjured up by human suggestion, proposition, or excitement.

The Pentecostal or Charismatic sees the baptism or infilling of the Holy Spirit as an experience subsequent to Christian conversion: one that comes about through a process of yielding the complete person into the guidance and indwelling of the Holy Spirit. We agree that the Holy Spirit is operative in every believer and in the varied ministries of the church. Still every believer must answer the question of Acts 19:2, "Have you received the Holy Spirit since you believed?"

Two expressions should be qualified here:

1. It should be understood that by "baptism in the Holy Spirit" the traditional Pentecostal/Charismatic does not refer to that baptism of the Holy Spirit accomplished at conversion, whereby the believer is placed into the body of Christ by faith in His redeeming work on the Cross (1Cor 12:13). Thus, no biblically oriented Charismatic ever views a non-Charismatic as "less saved" or less spiritual than himself.The baptism in or with the Holy Spirit (John1:33; Acts 1:5) was and is directed by the Lord Jesus to be "received" (John 20:22; Acts1:8) as a "gift" given following His ascension (John 7:39; Acts2:38.39). However should any prefer to dismiss this terminology, we contend that to experience the Holy Spirit's fullness in the spirit of unity is more important than to separate company or diminish our passion for His fullness over differences in theological wording or practice.

2. By "a process of yielding the complete person" the Pentecostal/Charismatic does not mean either (a) a passivity of mind or (b) a self-hypnotic or trance-like state. Rather, this terminology refers to an assertive prayerful, heartfelt quest for God. The mind is active, worshiping Jesus Christ, the Baptizer with the Holy Spirit (John 1:33). The emotions are warmed, as the love of God is poured forth into our hearts (Rom. 5:5). One's physical being participates, as worship is spoken and expressed, with upraised voice in prayer (Acts 4:24) or upraised hands of adoration (Ps. 63:1-5)

This is taken from the Spirit Filled Life Bible, pages 2019-2020 on Holy Spirit Gifts and Power.