Question: Can you explain what it is to be baptized in the Holy Spirit and the concept of being "slain in the Spirit". Does it hold any merit and have a legitimate backing in the Bible? What is the process and, for what it offers, are we able to achieve that another way than being supposedly "slain". Please give me some background.
Response: Baptism in (or with) the Holy Spirit is indeed legitimate and scriptural (cf. Matt.3:11; and see below), although it is very often mis-understood and confused with "baptism of (or by) the Spirit".
Baptism with water was a well-known symbol in Jewish ritual, and John "the baptist" directed by God used it to symbolize the washing away of sin through repentance and faith in God and the coming Messiah. John was very clear that it was "with fire and with the Spirit" that the Messiah would do His baptizing (Matt.3:11; Mk.1:7; Lk.3:16). The fire part speaks of the divine judgment Christ will visit upon the wicked of the earth at His return, through His rule, and at the end of the ages (cf. 2Thes.1:5-10). The baptizing with the Spirit speaks to the pouring out of the Holy Spirit on all who put their faith in Jesus Christ. All believers in Jesus Christ receive this "unction" or portion of the Spirit (cf. Rom.8:9), and it is this indwelling presence of the Spirit which empowers us (Gal.5:16), enlightens us (1Cor.2:6-16), comforts us (cf. Jn.14:15-21), and generally makes the Christian life the unique experience it is (cf.Gal.5:17).
It should also be pointed out that there is a difference between the gift of the Spirit we receive upon becoming one with Christ (which you have asked about) and the fact of our becoming one with Christ (which may be at the root of the second part of your question), a phenomenon which is also explained in the Bible via the concept of baptism (as indeed the two are inter-related). When we accept Jesus as our Savior, we are not only infused with the Spirit (i.e., the Spirit takes up residence within us), but we are also entered by the Spirit into Christ (Rom.6:3; cf. Matt.28:18-20), and henceforth are one with Him and abide in Him (as His "body" and "bride"). The first of these ministries is generally called "baptism in or with the Spirit" (where the Spirit is the subject who is poured into us rather than the agent who immerses us into Christ), while the second is most often referred to as "baptism by or of the Spirit" (where the Spirit is doing the work of baptizing us into Christ rather than being the "water" or "oil" that anoints us). In both cases, the analogy of water baptism is the point of reference which explains the concept. In the first case, just as when things are dipped into water and become wet, so we who have put our faith in Christ are forever soaked with His Holy Spirit (a notable characteristic of all believers which can be observed the way a wet person stands out in a crowd of dry ones). In the second case, just as when we plunge something into water it is subsumed by the water and sinks down into it, so we who have accepted Jesus are "immersed in Him" (i.e., we are His in an intimate and complete way, completely encompassed by Him). The Holy Spirit is intimately involved in both these aspects of "baptism", but differently in each case. In the former (with/in), He comes into us, while in the latter, He places us into Christ. The combination of these blessed occurrences means that we are, through His Spirit, in Him and He in us forevermore (cf. Jn.14:20: "I in My Father, and you in Me, and I in you"; cf. 1Cor.15:28). Here are some other links where I explain these matters in greater detail:
Baptism: Water and Spirit
Baptism and Following Jesus.
An Extended Conversation about the Baptism of the Holy Spirit.
The baptism of the Holy Spirit as distinct from speaking in tongues.
As far as being "slain in the Spirit" is concerned, though I have heard phrasings similar to this in the past, this is not scriptural language (i.e., you cannot find this term in the Bible). If I am not mistaken this phrase is generally associated with charismatic groups and with the individual concerned falling to the floor uncontrollably, ostensibly under the influence of the Spirit. However, there is no biblical basis for assuming that such behavior is of God; there are plenty of parallels in non-Christian religions and cults, however. I would advise giving all such obviously aberrant behavior a very wide berth. You might take a look at these links to Andrew Strom's Kundalini Warning for some disturbing parallels.
Part 2: http://www.youtube.com/user/JohnTheBaptistTV#p/a/u/1/BCcGaTRwG_4
It is true that as Christians, we have been "baptized into [the person of] Christ", and that this includes baptism "into His death" (Rom.6-7). That is to say, through the baptism of the Spirit, we have become Christ's own body, Christ's own bride, and therefore share all that He is: we shall be resurrected for Him, with rule with Him, and be together with Him forevermore. As those united to Christ, we have indeed died to this world. And since this reality of we believers being "dead to the world but alive to Christ" is accomplished by means of the Spirit [who entered us into union with Christ and dwells within each and every one of us now], it is possible that the terminology you have shared with me is meant to refer in some way to this special relationship believers have with our Lord Jesus, in particular to the fact that we are no longer in any way truly of this world, but rather have "died to it" in effect, by being made alive to the One who has overcome it. This is something that all believers have from the point of faith in Christ, and something we are called to put into practice in our lives (that is, by making choices for Christ just as if the world meant nothing to us and He meant everything ... which is the case!!!).
One other caveat is in order here. It is an occupational hazard of those who teach/preach to - occasionally [one hopes rarely] - build theology on theology rather than on scripture. In the case you present, I can sense "between the lines" of your question that there is possibly some rather strange application or "developed theology" lurking in this phrase given "slain in the Spirit". If this is taken to mean our death to the world through our participation in the death of Christ (union with whom is accomplished by the Spirit), well and good. If, however, something more, something "beyond what is written" is meant (especially if it involves some "process" of behavior which is not biblical), then "let the hearer beware". Here are some other important links which bear on these issues:
Combating Legalism VI
Combating Legalism V
Combating Legalism IV
Combating Legalism III
Combating Legalism II
Combating Legalism I
I hope this is of some help to you, and would be happy to clarify anything here about which you may have further questions.
Yours in Him through whom we have indeed died to the world but have become alive forever to the One who bought us, our Lord and Savior Jesus Christ!