One reason is because you and I are part of a much larger spiritual battle—a cosmic battle between God and the devil. The Bible warns, "For our struggle is not against flesh and blood, but against ... the spiritual forces of evil in the heavenly realms" (Ephesians 6:12).
Never forget: Satan is absolutely opposed to God, and his single goal is to stop God's work—including His work in our lives. When we come to Christ, we change sides (as it were), moving our citizenship from Satan's kingdom to God's Kingdom. Now our goal is to follow Christ and become more like Him.
But this doesn't mean Satan is going to leave us alone! He may even redouble his efforts once we come to Christ, trying to lure us back or get us to compromise our faith. The Bible says, "Your enemy the devil prowls around like a roaring lion looking for someone to devour" (1 Peter 5:8).
What is the solution? First, don't give the devil a toehold in your life! If old "friends" drag you down, stay away from them. If old habits have you in their grip, ask God to help you replace them with new ways of living. In addition, learn to walk with Christ every day through reading the Bible, prayer and fellowship with other believers. Finally, don't rely on your own strength to win the battle but on Christ and His strength.