“To thine own self be true” is not in the Bible. Whether it is a good motto to live by all depends on what one means by it.
“To thine own self be true” is from William Shakespeare’s Hamlet. It is part of the advice that the king’s advisor Polonius gives to his son Laertes before he leaves home for France. The fuller context (from Act I, Scene 3) will help:
“Give every man thy ear, but few thy voice;
Take each man’s censure, but reserve thy judgment.
Costly thy habit as thy purse can buy,
But not express’d in fancy; rich, not gaudy;
For the apparel oft proclaims the man,
And they in France of the best rank and station
Are of a most select and generous chief in that.
Neither a borrower nor a lender be;
For loan oft loses both itself and friend,
And borrowing dulls the edge of husbandry.
This above all: to thine own self be true,
And it must follow, as the night the day,
Thou canst not then be false to any man.”
There is a lot of good advice here, and much of it would be in agreement with biblical principles.
In context, Polonius is telling his son to be authentic and genuine, which is usually pretty good advice. If you don’t pretend to be something you are not, you will not appear false to other people. If you are a pretender, a poser, a faker, or a hypocrite, you may fool some people, but eventually you will appear false—you will be exposed for what you are. If you are genuine, you will never have to worry about being exposed. The Bible never specifically states this, but it does speak about the dangers of hypocrisy.
Although Shakespeare was not a Christian, as far as we know (and we know very little about the man other than his writings), he wrote when the culture was largely dominated by Christian values. Today, if someone takes “to thine own self be true” as his motto apart from the over-arching Christian worldview and biblical moral framework, it could become an excuse for almost any kind of bad behavior:
“I don’t love my wife anymore, but I do love my secretary. I need to be true to myself.”
“I was born a man, but I feel like a woman. I need to be true to myself.”
“I would spend more time with my kids, but I get so much more fulfillment by being successful at work. I need to be true to myself and do what I enjoy.”
“I really should get a job, but work is so boring. I think I will just hang out at the pool. I need to be true to myself.”
“I know I promised to help you, and at the time I meant it. But I just don’t feel like doing it anymore. I have to be true to myself, and I have changed my mind.”
“Being true to myself” without any context could mean that I will act sinfully or irresponsibly because it is what I really feel like doing. The Bible presents authenticity as doing what is true and right—being honest and genuine—even if you don’t feel like it or even if it is to your disadvantage.
Proverbs 15:4 speaks highly of the man who “who swears to his own hurt and does not change.” In this case, a person swears (promises) to do something or be truthful about something even if it is to his detriment. This is being genuine and authentic. However the modern application of “to thine own self be true” might be that, regardless of the truth of the matter or what you may have promised in the past, you have to protect yourself, and if you have to lie or break a promise to do it, that is OK because that is being authentic—that is “your truth.”
So, “to thine own self be true” is a fine motto, as long as it is understood within the context of a Christian worldview and a biblical moral framework. If you are a Christian, being true to yourself is really being true to the person you are in Christ and being true to what He has called you to be and do (Ephesians 4:1).