this is like your clothing question ..
you want to please God in all your behavior ..
so we don't advertise the flesh or engage in sexually suggestive things ..
I think a tongue piercing is suggestive ..
Jesus himself said, it is not what goes into your mouth that defiles you ..
OR ON YOUR ARM, OR IN YOUR BELLYBUTTON ..
but it is what goes into your heart that defiles you ..
so if in your heart you are acknowledging another god or do something that promotes sinful behavior, then it is sin ..
2Cr 5:17 Therefore if anyone is in Christ,
he is a new creature; the old things passed away; behold, new things have come.
Rom 12:1 Therefore I urge you, brethren, by the mercies of God, to present your bodies a living and holy sacrifice, acceptable to God,
which is your spiritual service of worship.
BIBLICAL PIERCING ..
Exd 21:6 then his master shall
bring him to God, then he shall bring him to the door or the doorpost. And his master
shall pierce his ear with an awl; and he shall serve him permanently.
Deu 15:17 then you shall
take an awl and pierce it through his ear into the door, and he shall be your servant forever. Also
you shall do likewise to your maidservant.
BIBLICAL MARKING ..
Eze 9:4 The LORD said to him, “Go through the midst of the city, even through the midst of Jerusalem, and
put a mark on the foreheads of the men who sigh and groan over all the abominations which are being committed in its midst.”
Rev 9:4 They were told not to hurt the grass of the earth, nor any green thing, nor any tree, but
only the men who do not have the seal of God on their foreheads.