If the LORD declares that our hearts are “more deceitful than anything else, and incurable” (Jeremiah 17:9), we can understand why He warns us to guard our hearts above all else. We can guard our hearts by being mindful of what we take in with our eyes and ears.
If a person who claims to be a Christian never studies God’s Word (the Bible), thinks prayer is irrelevant and forsakes regular church fellowship, their heart would be in danger, their spiritual life lacking and they would hardly be reflecting Jesus.
Read and study your Bible. Spend time in prayer daily. Gather with fellow Christians regularly. Allow the Holy Spirit to renew and regenerate you. Don’t partake in the ungodliness in this world. This is how we can guard our hearts as Christians.