Let me try to be a little more specific
If we are translated into His kingdom under his rule
Why is there sin also in His kingdom
How did it get there,
How can sin be in His kingdom
While the born-again person is freed from the penalty and power of sin, not until they move into eternity are they entirely free of the presence of sin also. And so, Christians are pressed on every side by the World, the Flesh and the devil with invitations to sin; not always in gross ways, like adultery, or murder, or theft, but in ways that are more subtle, philosophical ways, ways pertaining to values and life-goals, to relationship choices, to handling of money, to entertainment choices, etc.
Also, the Christian must contend with old sin-habits, only
over time winning free of them and in this
process of change developing new spiritual habits, forming new desires and understanding God's truth with ever-increasing, life-transforming depth. Though there is a spiritual metamorphosis that happens at conversion, there is no instantaneous change of their daily, mundane condition that happens for the person just converted. No, they must be altered
progressively by the Holy Spirit; and as they are, sin will still crop up in their life, though growing more and more the exception rather than the rule over time. And so it is that the NT speaks of spiritual growth:
Ephesians 4:13-15
13 until we all attain to the unity of the faith and of the knowledge of the Son of God, to mature manhood, to the measure of the stature of the fullness of Christ,
14 so that we may no longer be children, tossed to and fro by the waves and carried about by every wind of doctrine, by human cunning, by craftiness in deceitful schemes.
15 Rather, speaking the truth in love, we are to grow up in every way into him who is the head, into Christ,
1 Peter 2:2
2 like newborn babies, long for the pure milk of the word, so that by it you may grow...
2 Peter 3:18
18 But grow in the grace and knowledge of our Lord and Savior Jesus Christ. To him be the glory both now and to the day of eternity. Amen.
1 Corinthians 3:6-7
6 I planted, Apollos watered, but God was causing the growth.
7 So then neither the one who plants nor the one who waters is anything, but God who causes the growth.
Colossians 2:19
19 ...holding fast to the head, from whom the entire body, being supplied and held together by the joints and ligaments, grows with a growth which is from God.
A new Christian may suffer in their spiritual growth, remaining mired in carnality and sin, because of bad teaching and the absence of biblical discipleship. They may be led into a sensual sort of faith, or a legalistic one, or into Christian belief that is doctrinally-heretical; they may be encouraged to take up a hyper-emotional and thoughtless faith, poisoned and diseased in their walk with God by false teachers. And so on. (
Matthew 7:15; Mark 13:22; Acts 20:29; Jude; 2 Peter 2)
For these reasons (and others) sin may continue in a born-again person's life (and thus in God's kingdom). This is why in Scripture we read of believers who were carnal, fractious and selfish (
1 Corinthians 3, 5, 6, 11; Revelation 2-3), migrating into legalism (
Galatians 3:1-3), ignorant of their identity in Christ (
Romans 6), and also many warnings, and exhortations and criticisms given to the Early Church (
2 Corinthians 6:14-18; Ephesians 5:1-13; 2 Peter 2, Hebrews 5:12-14, etc.).