When I hear a believer announce, "I'm just a sinner," I feel like saying, "That's what you used to be." A lot of folks cling to a view of themselves as a patched-up, slightly-better-than-before version of their old self. The Bible contradicts that opinion: "If anyone is in Christ, he is a new creature; the old things passed away, behold, new things have come" (2 Cor. 5:17). In fact, according to Scripture, we're dramatically different once we are complete in Christ.
The question is whether people will trust in what they feel or believe what God says about them. His Word calls us saints (Rom. 1:7), disciples (Matt. 28:19), and fellow heirs with Jesus Christ (Rom. 8:17). If your opinion is that you are "just a sinner," then you cannot fully experience and enjoy your identity in Christ.
Believing what God says about our new self is a choice. Satan certainly conspires to convince believers that God's Word doesn't apply to them. He knows that people held captive by spiritual poverty back away from opportunities to share the gospel and serve the Lord's kingdom. It's much easier to spiritually bankrupt someone who already thinks of him- or herself as "just a sinner" than it is to conquer a disciple who knows God is his loving Father.
Our true identity is defined not by our past actions but by the Savior's. Jesus purchased our lives with His blood and brought us into relationship with God the Father, who adopted us as beloved children. We have every reason to hold our heads high, stand firm, and courageously proclaim the gospel.