613 Mitzvot
Negative Mitzvah 45
Not to cut yourself for the dead

Devarim (Deuteronomy) 14:1,2
Banim atem l'YHVH Eloheychem lo titgodedu velo-tasimu korchah beyn eyneychem lamet. Ki am kadosh atah l'YHVH Eloheycha uvecha bachar YHVH lihyot lo le'am sgulah mikol ha'amim asher al-peney ha'adamah.
You are children of YHVH your Elohim. You shall not cut yourself, nor make baldness between your eyes for the dead. For you are a holy people to YHVH your Elohim, and you He has chosen to be a treasured people unto Himself, above all the peoples that are on the face of the earth.
 
Vayikra (Leviticus) 19:28
Veseret lanefesh lo titnu bivesarchem uchtovet ka'aka lo titnu bachem ani YHVH.
And cuttings for the soul you shall not put in your flesh, and any marks you shall not put on yourself. I am YHVH.

This mitzvah starts out by saying, "You are children of YHVH." The Targum of Yonathan paraphrases it, "beloved children." It is written, "Behold, how great a love the Father has bestowed on us, that we should be called children of God!" (1 Yochanan/John 3:1). It is precisely this reason that we obey Him, as it is written, "By this we know that we love the children of God, when we love God and keep his mitzvot. {commandments}" (1 Yochanan/John 5:2).

YHVH has picked us out among the nations, to rise above them, to a special treasured people to Him, a holy nation.

Devarim (Deuteronomy) 26:18,19
and YHVH has declared you this day to be a people for his own treasured possession, as he has promised you, and that you should keep all his mitzvot; {commandments} and to make you high above all nations that he has made, in praise, and in name, and in honor; and that you may be a holy people to YHVH your Elohim, as he has spoken.

1 Kefa 2.9
But you are a chosen race, a royal priesthood, a holy nation, a people for God’s own possession, that you may proclaim the excellence of him who called you out of darkness into his marvelous light:

It is precisely for this reason, that YHVH doesn't want us to imitate the idolatrous practices of the nations. He wants us to be unique. One such practice he prohibits is cuting your flesh in mourning for the dead.

Keil & Delitzsch comments that the cuttings in the flesh "refers to passionate outbursts of mourning, common among the excitable nations of the East, particularly in the southern parts, and to the custom of scratching the arms, hands, and face, which is said to have prevailed among the Babylonians and Armenians (Cyrop. iii. 1, 13, iii. 3, 67), the Scythians (Herod. 4, 71), and even the ancient Romans (cf. M. Geier de Ebraeor. luctu, c. 10)"

Concerning mourning, "But we don't want you to be ignorant, brothers, concerning those who have fallen asleep, so that you don't grieve like the rest, who have no hope." (1 Thessalonians 4:13). There is a resurrection for those who hope in YHVH and His Messiah!

There is another application of this mitzvah. Chabad.org says concerning this mitzvah, "The Hebrew, Titgodedu (is used in the Torah with regard to this Negative Mitzvah. The word Aggudah comes from the same root-word and means 'group.' Therefore, our Sages explained that included in this Negative Mitzvah is the prohibition against separating into groups and the creation of division among our people."

1 Corinthians 11:18
For first of all, when you come together in the assembly, I hear that divisions exist among you, and I partly believe it.

This is not what the Messiah intends.

Yochanan (John) 17:21 that they may all be one; even as you, Father, are in me, and I in you, that they also may be one in us; that the world may believe that you sent me.