One argument used by weaker brethren against tattoos is that the Bible tells us not to conform to the world (e.g. Romans 12:2), and tattoos are worldly, therefore we should avoid them. Much like any argument against Christian Liberty, this argument falls short in that it uses a broad statement as if it is narrow. The assumption in the argument is that tattoos are necessarily included in "the world" to which we must not conform.
One clue that it isn't is that in almost every example (certainly every example I can think of off the top of my head) of the Bible telling us to not conform to the world also explains what it means by that. In Romans 12, for example, the context is clearly pride. "And be not conformed to this world: but be ye transformed by the renewing of your mind, that ye may prove what is that good, and acceptable, and perfect, will of God. For I say, through the grace given unto me, to every man that is among you, not to think of himself more highly than he ought to think; but to think soberly, according as God hath dealt to every man the measure of faith." (Romans 12:2-3). The passage continues to talk about Godly conduct, telling us to love one another, outdo one another in showing honor, being diligent, not seeking revenge etc. Nothing in the context of the passage indicates that Paul means "if the world does it, you shouldn't".
Indeed, in our every day lives, we conform to the ways of the world in which we live. If you're reading this article, you are benefitting from computer technology (even if someone has printed it out for you). So does the world. The chances are, even if you cannot personally drive, you have made use of some form of vehicle. So does the world. You are wearing clothes, presumably which in some way violate Levitical laws. So does the world. The very money with which you pay your bills, buy your food, and give to charity, is a very worldly thing. What's more worldly than making your way in life by passing out pictures of your country's figurehead?
So, clearly, the stronger brethren have the superior interpretation. Taking it to mean "the world does it, therefore we shouldn't" leads to absurd conclusions, including the idea that Christ Himself sinned when He asked for a denarius (Mark 12:15). But taking it to mean don't be ungodly, but rather seek Godliness, makes perfect sense.
This is the problem with using broad statements to prove narrow concepts. When we do this, we end up putting words in God's mouth, a sin which He warns us will lead to rebuke (Proverbs 30:6). In order to establish tattoos as sinful on the basis that they are worldly, one would first have to establish tattoos as being exclusively worldly. As it stands, however, only one verse in the entire Bible explicitly deals with the concept of tattoos: Leviticus 19:28. But though this does seem to condemn tattoos, it remains a poor argument against them, for reasons I briefly touched on earlier in this article.
As it stands, Christians are no longer under the law. To say otherwise, one must say that Jesus failed in His mission to fulfil it. But if you are inclined to such blasphemy, there is another sin you commit: Hypocrisy. See, those who seek to keep the law must keep the whole law (Galatians 5:3), not just the bit against tattoos. Scarier than that: Those who keep the whole law, but offend just once, are guilty with regard to all of it (James 2:10).
Those who oppose tattoos on the basis of the one and only explicit anti-tattoo verse in the Bible are condemning themselves in a number of ways. Have you eaten a bacon sandwich? Have you trimmed your sideburns? Are you still uncircumcised? These are all things which would violate the same law to which you appeal. Therefore, you are just as guilty as if you had a tattoo. And if you say "yes, but it's because I'm a sinner, not because I agree these things are ok", then fair enough, but you're still contradicting the numerous Scriptures that tell us we are no longer under the law.
So, we are indeed no longer under the law, eliminating the one explicit verse against tattoos in the Bible. That's not to say the verse is not important, God gave it to us for a reason. And of course, it really does prohibit pre-Christian Jews from getting tattoos. But it is not sufficient to establish tattoos as a sin for all people, for all time.
Because it is an example of covenantal law, the only example of an anti-tattoo verse in the Bible cannot be used to establish tattoos as worldly. But it is the only option. Nothing else in the entire Bible is so obviously against tattoos that there can be only one Christian view. To really drive this point home, let us compare other moral issues. Can I take a knife and stab some guy on the street today? "...no murderer has eternal life abiding in him." (1 John 3:15b). Well, can I steal his wallet then? "Let him who stole steal no longer, but rather let him labor, working with his hands what is good, that he may have something to give him who has need." (Ephesians 4:28). Ok fine, but you know, he's kind of cute. Can I ask him on a date and maybe sleep with him? "Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived. Neither fornicators, nor idolaters, nor adulterers, nor homosexuals, nor sodomites, nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners will inherit the kingdom of God." (1 Corinthians 6:9-10).
I think I've made my point. When God wants us to believe a thing is sin, He tells us. According to 2 Timothy 3:16-17, the word of God fully equips us for every good work. Therefore, if tattoos were truly sinful, the Bible would give us everything we need to conclude as much, to the extent where all but the most rebellious of Christians would disagree. Instead, because scripture does not address tattoos, we are to conclude that this is a disputable matter (Romans 14:1). That is, we can afford to have differing opinions.
If you are a Christian, and you are fully convinced in your own mind that tattoos are ok, you have the right to get one, but not to offend the weaker brethren. If you are a Christian but are not convinced tattoos are ok, then do not get one, for whatever does not come from faith is sin (Romans 14:23). But to judge your stronger brethren as being somehow worldly usurps the authority of God.
As Christians, let us unite around Christ, not judging each other in petty squabbles. Where scripture is ambiguous, we can afford to differ in our opinions, placing God in His proper place as Lord; our Commander and redeemer. Seek to please Him first, each other second, and ourselves third. Division over trivial issues is what the devil seeks, but unity of the whole Church is what he fears.