When you accept Jesus as your Lord and Saviour, the Holy Spirit, comes into your spirit, and becomes one with your spirit. The Holy Spirit, makes your dead spirit, alive, and so your spirit becomes a new creation. When you spirit is born again, it means that the Holy Spirit is permanently living in you, and is one with your spirit. Your born again spirit is 100 % perfect, as being a new creation, and therefore, can not sin anymore, and it does not want to sin anymore, because the Holy Spirit lives in your spirit. But, your soul - mind, intelect, is not born again, and is not perfect, like your born again spirit. So, your mind, has to be transformed and renewed, by the Word of God, in order, to align and agree, with your born again spirit, and to become more like your born again spirit. That is why, we need to read the Word of God.
Sin, and the sinful nature, live in your body. So now you have your perfect born again spirit, fighting and struggling against your mind, which needs transforming by the Word of God, and also, fighting against the sinful nature, which lives in your body.
That is why, the christian life, is a daily struggle.
Scripture reference;
John 3:6, 1 Corinthians 1:8, 6:17,19, Romans 8:10,11, 13, Psalm 36:1, 2 Peter 1:4, 2:22, Ephesians 1:19,20, Colossians 1:27,29, John 6:63, 14:6, John 1:4, 3:16, 4:10,14, 5:26, 15:1-6 6:27,33,39,47,48,51,53,54, 7:38, 10:10, Acts 17:25, Ecclesiastes 7:20, Hebrews 4:12, 2 Corinthians 3:6, 4:10-13, 5:4, Matthew 4:4, 7:15-23, 23:27,28, Revelation 21:6, 22:1,2,15,17,19, Galatians 6:8, Colossians 2:13, Ephesians 4:18,24, Acts 3:15, Romans 8:6, 1 Timothy 6:12, 1 John 1:1,2, 3:15, 4:7, 13, Proverbs 2:14, 3:18, 21:10, Psalm 16:11, John 15:1-8, Mark 4:6-16,17,18,19, Galatians 5:22, Philippians 4:7, John 15:10, Acts 5:38, 39, John 3:3, Romans 8:16, John 3:5, 6, Genesis 2:17, John 10:10, Proverbs 10:23, Acts 1:8, 1 Corinthians 12:3, 2 Thessalonians 2:12, Titus 1:16, James 1:18, 2:14, 1 Peter 1:23, 2 Peter 1:4, 1 John 2:19, 3 John 11, Hebrews 12:4, Proverbs 4:16,23, 12:10, 16:6, Hebrews 12:14, Psalm 22:26, Luke 10:25-28, 13:23,24 , Isaiah 26:10