Do I believe that a Born Again, New Creature in Christ, a Christian who is justified by faith can lose their salvation?
No, I do not! The Scriptures are very clear on the idea that we are kept by God from falling.
Check out the following verses. They show that God's people are given ETERNAL life the moment they believe (if we could lose our salvation it wouldn't be eternal). They show that we are kept by God's power and nothing can separate us from His love. They show that we are sealed with the Holy Spirit who has been given as a guarantee of our eternal salvation and inheritance: Isaiah 43:1-3; Isaiah 54:10; Jeremiah 32:40; Matthew 18:12-14; John 3:16; John 3:36; John 5:24; John 6:35-40; John 6:47; John 10:27-30; John 17:11,12,15; Romans 5:8-10; Romans 8:1; Romans 8:29-30; Romans 8:35-39; 1 Corinthians 1:7-9; 1 Corinthians 10:13; 2 Corinthians 4:14,17; Ephesians 1:5,13,14; Ephesians 4:30; Colossians 3:3-4; 1 Thessalonians 5:23-24; 2 Timothy 4:18; Hebrews 9:12,15; Hebrews 10:14; Hebrews 12:28; 1 Peter 1:3-5; 1 John 2:19,25; 1 John 5:4,11-13,20; Jude 1; Jude 24-25.
I realize there are verses in the Bible that seem to teach otherwise (examples Matthew 7:21-22; Luke 8:1-15; Hebrews 6:1-8), but these verses are very few compared to the overwhelming amount of verses on our eternal security. And I believe in every case the verses that seem to teach otherwise are about people who PROFESS to believe, but don't actually believe. (See for example Matthew 7:23, Jesus says "I NEVER knew you.")
There is a difference between the person who answers an altar call because of guilt, emotion, or social pressure and the person who actually commits their life to the Lord from their whole heart. The former is only a professor. The latter is a true believer.
Can a Christian sin and still be saved?
Absolutely! The apostle John said, "If we claim to be without sin, we deceive ourselves and the truth is not in us... If we claim we have not sinned, we make Him out to be a liar and His word has no place in our lives" (1 John 1:8,10). James, the Lord's half-brother, said, "We all stumble in many ways" (James 3:2). Paul said, "Christ Jesus came into the world to save sinners--of whom I AM the worst" (1 Timothy 1:15).
I don't believe that sin causes us to lose our salvation. Sin is serious, there are consequences for our sin, and God will discipline us because He loves us enough to want us to change, but a Christian has a whole new relationship to sin. Take for example, your relationship with your earthly father. If you disobey him, you are not a good daughter. If you rebel against him, you cause him much pain. But YOU ARE STILL HIS DAUGHTER. Nothing can change that fact. Likewise, if you truly have given your life to Christ, then you are born of God. You have a heavenly Father. Nothing can change that. For more see What Sin Does and Doesn't Do in the Life of a Christian. My experience with people who believe in sinless perfectionism is that they are usually strong on the outward appearances (like what they wear, how often they go to church, etc.). They are also strong on what they or others SHOULDN'T do, i.e. the DON'Ts of the Christian life. They are usually also very critical people (of themselves and others), because in their view salvation is based on being perfect.
But my experience with people who believe in perfectionism is that they are weak on the heart issues that Jesus is so concerned about, such as humility, love, joy, compassion, forgiveness, forbearance, and peace.
I like Paul's statement: "For the kingdom of God is...righteousness, peace, and joy in the Holy Spirit, because anyone who serves Christ in this way is pleasing to God and approved by men" (Romans 14:17-18). When I look for a mature believer I look for someone who is in right relationship with God, full of peace rather than worry and fighting and judging, and full of joy! This is what pleases God, not the emphasis on debatable legalisms and spiritual competitions with one another.
This is my opinion, but my opinion is not really important. Study God's Word and let Him guide you. He is a loving Father and rewards anyone who seeks Him and wants to know what is true. He will direct you. When you are sure of what your Father tells you, then hold on to it with all your heart and don't let anyone disturb your peace. Your Father has spoken and that's enough.
