Speaking in tongues was considered by N.T. Christians as a God-given sign of baptism in the Holy Ghost. This Biblical pattern is still valid for us today.

TRUE SPEAKING IN TONGUES

1. Speaking in tongues is a supernatural manifestation of the Holy Spirit where the believer speaks in an unknown language(Acts 2:4; 1 Cor. 14:14-15). It is not "estatic speech" as some claim, for the Bible never uses the term "estatic utterance" to refer to speaking in tongues.
2. Tongues is the outward sign of baptism in the Holy Ghost. God linked tongues with the Holy Ghost baptism from the very start(Acts 2:4), so that the 120 believers at Pentecost and all thereafter, would have an external confirmation that they have indeed received the Holy Ghost. Throughout the history of the church, whenever tongues as a sign has been denied, the truth and experience of Pentecost has declined, been distorted, or been ignored entirely.
3. Tongues is described as a gift given by the Holy Spirit to the believer(1 Cor. 12:4-10). It has two main purposes: (a) tongues, along with the interpretation, in a worship service will edify the congregation into a more Spirit-directed praise(1 Cor. 14:5-6, 13-17), (b) tongues is used by the believer to speak to God in his personal devotions and therefore, build up one's spiritual life(1 Cor. 14:4).

FALSE SPEAKING IN TONGUES

1. The mere occurence of "other tongues", or any other supernatural manifestation, is not absolute evidence of the work and presence of the Holy Spirit. Speaking in tongues can be imitated by human initiative or demonic activity. The Bible warns us not to believe every spirit, but examine whether they are from God or not(1 John 4:1).
2. Other tongues, in order to be valid, must be "as the Spirit gives utterance"(Acts 2:4). They must be the spontaneous result of the initial filling of the Holy Ghost. It is not a learned phenomenon, nor can it be taught to believers by speaking incoherent syllables.
3. The Bible warns that in the last days there will be, within the church, hypocrisy(1 Tim. 4:1-2), signs and wonders from satanic powers(Matt. 7:22-23; 2 Thess. 2:9), and deceitful workers disguising themselves as believers(2 Cor. 11:13-15). We must take heed as these relate to fake manifestations and signs(Matt. 7:22-23; 2 Thess. 2:8-10).
4. To discern whether someone's speaking in tongues is genuine, we must look for the Biblically defined results of the baptism in the Holy Ghost(see Proof of Genuine Baptism in the Spirit). If someone claiming to speak in tongues is not committed to Christ and is not obeying the Word of God, they are not of God(1 John 3:6-10; 4:1-3; Matt. 24:11,24; John 8:31; Gal. 1:9).