One of the most important things Christians need to do is think about what is true. These are the things that do not change with time, culture, circumstances, or emotion - things that cannot be compromised or minimized. We need these things to be always fresh in our minds and written on our hearts, so we can recognize lies and deceit when they try to attack. Some verses come to my mind that remind me that truth is something we fight for and defend:
"We destroy arguments and every lofty opinion raised against the knowledge of God, and take every thought captive to obey Christ..." (2 Corinthians 10:5)
"See to it that no one takes you captive by philosophy and empty deceit, according to human tradition, according to the elemental spirits of the world, and not according to Christ." (Colossians 2:8)
These verses both talk about captivity - either our thoughts are taken by lies, or submitted to Christ. The first result happens when we let our guard down, but the second takes great discipline. It means making Scripture the authority in our lives, and not picking and choosing the pieces that make us feel good. It means reminding ourselves daily of the attributes of God, even when we don't always see them working in the way we want them to. It means remembering who we once were in darkness, and who we are now, because of Jesus Christ. It means rehearsing the Gospel to ourselves and to each other often - it is not just for those who are lost, but it is the ongoing, life-transforming, sanctifying truth for us who have been rescued... and you will find that when you meditate on these thoughts, they turn into praise to the Lord - Hallelujah, what a Savior!
Whatever is true... think about these things.