Jeremiah 17:9
“The heart is deceitful above all things, And desperately wicked; Who can know it?
Romans 3:23
... for all have sinned and fall short of the glory of God,
Romans 6:23
For the wages of sin is death, but the gift of God is eternal life in Christ Jesus our Lord.
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Are we still desperately wicked in our hearts after we become Christians?
Yes, we are still wicked in our hearts even after we become Christians.
Biblically speaking, the unbeliever is completely and totally affected by sin in every area of his being: heart, soul, mind, body, emotions, etc. In this sense he is desperately wicked (though not all unbelievers are as bad as they can be in the manifestation of that wickedness). This is why the Bible says that man is deceitful (Jer. 17:9), full of evil (Mark 7:21-23), loves darkness rather than light (John 3:19), does not seek for God (Rom. 3:10-12), is ungodly (Rom. 5:6), dead in his sins (Eph. 2:1), by nature a child of wrath (Eph. 2:3), cannot understand spiritual things (1 Cor. 2:14), and is a slave of sin (Rom. 6:16-20).
However, when we become regenerate, we are new creatures in Christ (2 Cor. 5:17). Yet, we’re also still desperately wicked. We won’t be delivered from this until we receive our new bodies in the future resurrection (1 Cor. 15:35-45). For now, we still live in fallen flesh and struggle with the sin nature that affects all of who we are. It is in the battle between our old sinful selves and our new redeemed selves that our victories overcoming sin and failures in sin manifest. So, even as Christians, we are still touched by sin in all areas of who we are; hence, we are desperately wicked—in that we are still completely touched by sin. The difference is that we also now struggle against our wickedness because we have been changed by God. We have been made new creatures; and because of that work of God, we can now recognize our sin and seek to turn from it where the unbeliever will not.
I think the words of Paul the apostle are appropriate here:
“For the good that I wish, I do not do; but I practice the very evil that I do not wish. 20 But if I am doing the very thing I do not wish, I am no longer the one doing it, but sin which dwells in me. 21 I find then the principle that evil is present in me, the one who wishes to do good. 22 For I joyfully concur with the law of God in the inner man, 23 but I see a different law in the members of my body, waging war against the law of my mind, and making me a prisoner of the law of sin which is in my members. 24 Wretched man that I am! Who will set me free from the body of this death? 25 Thanks be to God through Jesus Christ our Lord! So then, on the one hand I myself with my mind am serving the law of God, but on the other, with my flesh the law of sin,” (Romans 7:19-25).
https://carm.org/are-christians-still-wicked