IS TATTOO A SIN?
The Holy Bible is final authority on any matter of Christian faith. It's not pastor's opinion or any school of thought that matters.

What's tattoo? It is print on skin.
Leveticus 19 28
“Ye shall not make any cuttings in your flesh for the dead, nor print any marks upon you: I am the LORD.”

1 John 2:15–17 (KJV)
“Love not the world, neither the things that are in the world. If any man love the world, the love of the Father is not in him.”
“For all that is in the world, the lust of the flesh, and the lust of the eyes, and the pride of life, is not of the Father, but is of the world.”
“And the world passeth away, and the lust thereof: but he that doeth the will of God abideth for ever.”

What's sin? Sin is transgressions of law of God. Tattoo is sin according to the above scriptures.
This is not judgment but clarifying doubt whether tattoo is a sin or not.
Bible is final authority authority on any matter in Christian faith therefore tattoo is a sin.

Shalom