Is a believer's heart good or evil?

from the old testament

• After humanity rebelled against God, our "hearts" were also corrupted (emotions, wills, minds, and so forth). This is why we are all born sinful (Psalm 14:2-3, 51:5).
• Jeremiah describes this deep internal corruption: “The heart is deceitful above all things, and desperately sick; who can understand it?” (Jeremiah 17:9). Even God’s chosen people in the Old Testament, the Israelites, struggled with this wickedness—despite God dwelling among them (Exodus 25:8) and instructing them in His ways (Deuteronomy 10:12–13), they were unable to fully obey and please Him.
• This is why God promised to one day “give you a new heart, and a new spirit I will put within you. And I will remove the heart of stone from your flesh and give you a heart of flesh. And I will put my Spirit within you, and cause you to walk in my statutes and be careful to obey my rules” (Ezekiel 36:26–27).

from the new testament

• Jesus also noted that sin flows from the heart: “What comes out of a person is what defiles him. For from within, out of the heart of man, come evil thoughts, sexual immorality, theft, murder, adultery, coveting, wickedness, deceit, sensuality, envy, slander, pride, foolishness. All these evil things come from within, and they defile a person” (Mark 7:21–23).
• Though all people, even believers, still sin because we are born with a sin nature (1 John 1:8), when we are saved, God imputes Jesus’ righteousness to us. Christ's death on the cross was sufficient to cover believers' guilt (Romans 3:25–26). God transfers the guilt from the sinner to Jesus, and Jesus’ righteousness to the sinner.
• Believers who are saved still sin, but they are legally justified. That is, before God we are seen as not guilty, as if we had not sinned. We become new creations (2 Corinthians 5:17), so we are no longer slaves to sin (Romans 6:6). This does not mean believers do not sin but that we are no longer under sin’s dominion and are now empowered to resist it and grow in righteousness (Romans 6:12–14; Galatians 5:16–17). Though the struggle remains, believers have the Holy Spirit, who actively renews and transforms us from the inside out (Romans 8:9; Titus 3:5; 2 Corinthians 4:16).
• The Holy Spirit (Romans 8:9) cleanses believers from the inside out (Titus 3:5). He sanctifies us and internally renews us (2 Corinthians 4:16; cf. Romans 12:2).
• This doesn't completely eliminate believers' struggle with sin. For example, in Romans 7:18–25 the apostle Paul describes his own battle with sin (cf. 1 John 1:8, 10). This is why elsewhere Paul would say, “I say, walk by the Spirit, and you will not gratify the desires of the flesh. For the desires of the flesh are against the Spirit, and the desires of the Spirit are against the flesh, for these are opposed to each other, to keep you from doing the things you want to do" (Galatians 5:16–17).
• Believers will continue to sin until they see Jesus (1 John 3:2), at which point we will be fully transformed to be like Christ, including renewed hearts (Philippians 3:20–21).
• Even in this wrestle with sin, believers can rest assured that “[the Father] who began a good work in you will bring it to completion at the day of Jesus Christ” (Philippians 1:6).

implications for today

Believers are in a battle . . . an internal one, against sin. Though believers have given their life to Christ, have died to sin and the old self, and have been made alive to righteousness and the things of God, we will still fight the flesh and sin. We do not have perfected hearts, but, because our hearts are no longer enslaved to sin (Romans 6:6) and the Holy Spirit lives within us, we can live for God and not for sin. Believers are not saved to drift through this life passively but to actively be like Jesus, being lights in this world (Matthew 5:14–16). We are called to do the "good works, which God prepared beforehand, that we should walk in them" (Ephesians 2:10). The battle is real, but the victory is certain, because the One who saved you is also the One who will finish the work in you.