The Word of God in Leviticus 19:28 says,
“Ye shall not make any cuttings in your flesh for the dead, nor print any marks upon you: I am the LORD.”
I Corinthians 3:16–17 says,
“Know ye not that ye are the temple of God, and that the Spirit of God dwelleth in you? If any man defile the temple of God, him shall God destroy; for the temple of God is holy, which temple ye are.”
And I Corinthians 6:19–20 says,
“What? know ye not that your body is the temple of the Holy Ghost which is in you, which ye have of God, and ye are not your own? For ye are bought with a price: therefore glorify God in your body, and in your spirit, which are God's.”
The Word of God is very clear in saying that we are NOT to print any marks upon our bodies.
As Christians, our bodies are the temples of the Holy Spirit which we have of God, and we are not our own. We are to glorify God in our bodies.
Therefore, it IS a sin to GET a tattoo.
However, it is not necessarily a sin to HAVE a tattoo on one's body. If a person received a tattoo or tattoos prior to accepting Christ as their Lord and Savior, the sin of getting their body tattooed was washed away by the blood of Jesus when the person accepted Christ as their Lord and Savior.
If they received a tattoo or tattoos subsequent to accepting Christ as their Lord and Savior, and they have since repented of and confessed the sin of getting their body tattooed, then they are no longer in sin although the tattoo or tattoos are still on their body.
In some ways this is similar to the sin of gluttony. The sin of gluttony, or eating too much, can cause a person to be fat (overweight). But being fat, in and of itself, is NOT a sin. The sin was committed when the person ate too much food. However, the fat person may or may not be in sin. If the person has become a Christian since committing the sin of gluttony, the sin of gluttony was washed away by the blood of Jesus when they became a Christian. If they committed the sin of gluttony subsequent to becoming a Christian, and the person has repented of and confessed the sin of gluttony, they are no longer in sin—even though they are still fat.
Therefore, we cannot judge whether or not a person is in sin if they have a tattoo or are fat.
However, a person who has a tattoo or tattoos, or a person who is overweight, should be praying about and working on getting rid of the tattoo or tattoos and/or the excess weight.
Sometimes in toy stores and at fairs and carnivals and things “temporary tattoos” are sold or given away. These are “tattoos” that look like real tattoos but are only on the outside of the skin and can be easily washed off. There is nothing wrong with a person—even a child—getting these kinds of “tattoos” because they are not permanent marks upon their flesh.