Introspection is not pleasant for man. It is much easier for us to look outward and point to the issues in other's lives. Jesus addresses this when he says that we need to get the log out of our eye before pointing out the speck in our brother's eye. To point out a problem is easy, but to see issues in your own life means you have to deal with them. Introspection means that you have to deal with what you find in your life. You cannot pass by it.
The Bible says, "Keep your heart with all vigilance, for from it flow the springs of life." (Proverbs 4:23 ESV) Charles Bridges, in his commentary on the book of Proverbs, says the heart is "the citadel of man" and "the seat of his dearest treasure." Introspection needs to start with your heart. The heart is the key to your soul and the source of your actions. It, above all else, must not be lost, or all is lost.
In the game of Chess, the king is the crucial piece. Unlike checkers, where every piece has equal value and can become a king, there is one king in Chess whose protection is key to the game. In life, the security of your heart is crucial above all else.
Bridges says, "heart must be known in order to be effectively kept." How true, for if we are to guard something, and at that vigilantly, then it must be known to us. The heart can be a tricky thing. The Prophet Jeremiah says that the heart is deceitful above all things. Jesus tells us the importance of the heart in Matthew 15:17-20:
 Do you not see that whatever goes into the mouth passes into the stomach and is expelled?  But what comes out of the mouth proceeds from the heart, and this defiles a person.  For out of the heart come evil thoughts, murder, adultery, sexual immorality, theft, false witness, slander.  These are what defile a person. But to eat with unwashed hands does not defile anyone." (ESV)
It is from the heart that our speech and actions flow. Christianity is not a religion of rules and outward obedience. It is a religion of the heart. It has, at its core, the changing of the heart by God himself. So the first thing you need to know, has God changed your heart? Has the old, hard, and dead heart been replaced by the heart of flesh that is alive to God? This knowledge is vital to know! What good is it to keep a heart that is dead?
There are several signs that you have a new heart in Christ. Your heart should desire to worship God. It should hate sin. Its desire should be to follow God and live for God. The Proverb says it is from the heart that springs of life flow. Does the Christian life flow from your heart?
If you know this to be true in your life, then you must guard your heart from the world that would try and seduce it to sin and from the devil who would desire to destroy it. We have to be vigilant not only because of the outward threats but the ever-present internal threat of our sinfulness.
The flippancy of many Christians should concern us. Yes, we believe in the perseverance of the saints. Yes, we believe that if God holds us in His hand, no one can snatch us out of it. However, the Word of God tells us that we must keep our hearts and be vigilant about it. Should that not cause us to take the keeping of our hearts seriously? You should know the condition of your heart. You should know where it is weak and where it is healthy so that you can lean into your strengths and give a particular watch where there are weaknesses and dangers.
Take some time and consider your heart. Look with introspection at the key to your soul. What do you find in your heart? Where is your treasure? What words and actions are flowing from the spring of your heart? And what does this say about your heart's condition?