I was not familiar with this expression and had to look it up on the Internet.  One site explained it like this: “Slain in the Spirit or slaying in the Spirit are terms used by Pentecostal and charismatic Christians to describe a form of prostration in which an individual falls to the floor while experiencing religious ecstasy. Believers attribute this behavior to the power of the Holy Spirit. Other terms used to describe the experience include falling under the power of the Spirit, overcome by the Spirit, and resting in the Spirit. The practice is associated with faith healing because individuals are often ‘slain’ while seeking prayer for illness.”

It is really hard to define this act because it is not found in the Bible, therefore it could mean different things to different people, depending upon their understanding of what being slain in the Spirit implies. 

While this expression is not found in the Bible, there are other words that should encourage us to be fully led by the Holy Spirit.

Ephesians 1:13, “In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that holy Spirit of promise.”  This shows us that when we accept the Lord Jesus Christ as our savior, we are sealed with the Holy Spirit.  We also read in Romans 8:9, “But ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his.”  If someone is not indwelt with the Spirit, this shows he does not belong to Christ…he is not a believer.  (Read also Romans 8:14).

Ephesians 5:18, “And be not drunk with wine, wherein is excess; but be filled with the Spirit.”  Here we are taught that the Lord does not want us to be controlled by alcohol, rather He wants us to be controlled by the Holy Spirit by being filled with the Holy Spirit.  How do we fill ourselves with the Holy Spirit?  It is to allow the Spirit to influence, guide, and control us.  It is to be a faithful student of God’s Word and allow the Spirit to teach us. The Lord Jesus spoke of the purpose of the Holy Spirit in our lives in John 14:26, “But the Comforter, which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you.”  The Spirit teaches us by opening our understanding of the scriptures when we spend time in study.

Ephesians 4:30, “And grieve not the holy Spirit of God, whereby ye are sealed unto the day of redemption.”  We grieve the Spirit when we do those things He does not want us to do.

1 Thessalonians 5:19, “Quench not the Spirit.”  We quench the Spirit when we do not do those things He wants us to do. 

Galatians 5:16, “This I say then, Walk in the Spirit, and ye shall not fulfil the lust of the flesh.”  If we walk, or live, according to the leading of the Spirit, we will have the power to overcome the temptations of the flesh and will not seek to fulfill the lusts of the flesh.

The Lord does want us to be controlled by the Holy Spirit.  We can do that by submitting our lives to the Lord and allowing Him prominence in our lives.  In Revelation 1:10-11, John the Apostle declared that he was, “…in the Spirit on the Lord’s day…”  This shows he was under the influence and guidance of the Spirit; thus he was able to receive great prophecies from the Lord.  Let us be careful about allowing others to work us into a frenzy and convince us we are “slain in the Spirit” when we are just under the influence of our excitable and ecstatic flesh. When we are truly filled with the Spirit, He will manifest Himself in us with a quiet, submissive, joyous, and thankful character.  (CC)  (565.4)