Speaking in Tongues

New Testament References

The first place tongues is mentioned in the New Testament is in Mark 16:16-17:

And these signs shall follow them that believe;
In my name shall they cast out devils;
they shall speak with new tongues;

They shall take up serpents;
and if they drink any deadly thing, it shall not hurt them;
they shall lay hands on the sick, and they shall recover.
Mark 16:17-18

From this passage, we do not understand the context in which it is being used. The best way to study the meaning of a specific word is to look at the Greek word, and see where how it is used in other passages in the Scriptures. The word "tongues" here is from the Greek word "glossa" (Strongs Number 1100). BlueLetterBible.org defines "glossa" as:

of uncertain affinity.

the tongue, a member of the body, an organ of speech
a tongue

the language or dialect used by a particular people distinct from that of other nations

There are fifty instances of the word "glossa" in the New Testament:

In I Corinthians 14:21, Paul references an Old Testament scripture referring the word "tongues":

In the law it is written,
With men of other tongues and other lips
will I speak unto this people;
and yet for all that will they not hear me,
saith the Lord.
1 Corinthians 14:21

Old Testament References

This verse refers back to Deuteronomy 28:49. In Deuteronomy 28:49, the word tongue is derived from the Hebrew word "lashown" or "lashon" (Strong's number 03956) which is translated ninety-eight times as tongue, 10 times as language (Nehemiah 13:24, Esther 1:22 [4 times], Esther 3:12, Esther 8:9 [4 times], Jeremiah 5:15, Ezekiel 3:5, Ezekiel 3:6, Zechariah 8:23), 3 times as bay (Joshua 15:2,5, Joshua 18:19), 2 times as wedge (Joshua 7:21), 1 time as babbler (Ecclesiastes 10:11) and 1 time as flame (Isaiah 5:24), and 1 time as talker (Ezekiel 36:3):

tongue

tongue (of men)

tongue (literal)
tongue (organ of speech)

language
tongue (of animals)
tongue (of fire)
wedge, bay of sea (tongue-shaped) Joshua 15:2, 5

Tongue in the context of a language

We now want to just look at where the word "tongue" is used in the context of being a language:

The LORD shall bring a nation against thee from far,
from the end of the earth, as swift as the eagle flieth;
a nation whose tongue thou shalt not understand;
Deuteronomy 28:49

Children in whom was no blemish, but well favoured, and skilful in all wisdom, and cunning in knowledge, and understanding science, and such as had ability in them to stand in the king's palace, and whom they might teach the learning and the tongue of the Chaldeans.
Daniel 1:4

In this context, "tongue" is a foreign language. If we apply this to Acts, we see the same trend that the word "tongue" is used "the language or dialect used by a particular people distinct from that of other nations"

And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance.

Now when this was noised abroad, the multitude came together, and were confounded, because that every man heard them speak in his own language.

Cretes and Arabians, we do hear them speak in our tongues the wonderful works of God.

Acts 2:4,6,11

Controversial Verses

There are three particular verses that seem to cause much disagreement in the church about what they mean. They are:

For he that speaketh in an unknown tongue speaketh not unto men, but unto God: for no man understandeth him; howbeit in the spirit he speaketh mysteries.
1 Corinthians 14:2

Wherefore let him that speaketh in an unknown tongue
pray that he may interpret.
1 Corinthians 14:13

Therefore if I know not the meaning of the voice, I shall be unto him that speaketh a barbarian, and he that speaketh shall be a barbarian unto me.
1 Corinthians 14:11

The gift of speaking/interpreting tongues

Then we also must consider that speaking in tongues is a gift:

To another the working of miracles;
to another prophecy;
to another discerning of spirits;
to another divers kinds of tongues;
to another the interpretation of tongues:
1 Corinthians 12:10

This may mean that some believers are particularly gifted in learning and speaking languages of other nations. Also, as in the case of Acts, it appears there is a gift of being able to speak or understand languages or dialects from other nations in a supernatural way.

Maintaining unity in the body

Although believers may disagree on the meaning of these verses, we must remember that Jesus desire is that the church "that they may be made perfect in one; and that the world may know that thou hast sent Me" John 17:23. Therefore, we must overcome any differences with love:

Let us therefore follow after the things which make for peace,
and things wherewith one may edify another.
Romans 14:19

But avoid foolish questions, and genealogies, and contentions,
and strivings about the law; for they are unprofitable and vain.
Titus 3:9

References:

BlueLetterBible.org