"Beware, brethren, lest there be in any of you an evil heart of unbelief in departing from the living God, but exhort one another daily, while it is called today, lest any of you be hardened through the deceitfulness of sin." (Hebrews 3:12-13)
Sometimes we are lured away from God through the subtle deceitfulness of the world. Certain activities may seem harmless at first, and they probably are, as long as it is in moderation. It's the slow, inch-by-inch influence that can can be so dangerous.
For instance, friends with different mindsets, rated R movies that exploit the opposite of biblical principles, music that keeps repeating the same concepts that go against our beliefs--these things in moderation will not change our thought patterns, because of our biblical foundation and other Godly friends who will remind us. This is why it is so important to surround yourself with Godly friends and exhort one another daily. Also, time in God's word will help us keep focused on Him, but if we don't have a balance, the deceitful things of the world will begin to influence us no matter how much we think otherwise and deny it.
Godly friends can help us see when we're changing in ways that may not be pleasing to God. Our trust, friendship and history with them gives us reason to pause and reflect that perhaps there is some real viability in what they're saying. This is where teenagers often lack maturity in being able to withstand admonishment--even some adults. Godly correction is good for us and gives us a chance to correct our behavior before serious consequences result. So be sure to surround yourself with Godly people so you can have a trusted accountability partner--for me--that person is my husband.