Once saved always saved
A comparison of 1John 1:10 with 1John 3:9 explains how God prevents a Christian from losing his salvation. A born again Christian has two natures: a sin nature in the outward flesh and a sinless nature in the inward man spiritually. By making the inward, spiritual man sinless through the born again experience, God prevents spiritual death. If you can't sin, you can't die. Therefore, once saved, always saved.