Salvation is a gift; but like any gift, it can be lost through negligence. If you were given a free car, but neglected it by leaving the doors unlocked then the thief would come along and steal your free gift. This is much like Satan coming along and stealing your free gift of Salvation. Peter illustrates this point in what we should do to make our calling and election sure.
2 Peter 1
[5] And beside this, giving all diligence, add to your faith virtue; and to virtue knowledge;
[6] And to knowledge temperance; and to temperance patience; and to patience godliness;
[7] And to godliness brotherly kindness; and to brotherly kindness charity.
[8] For if these things be in you, and abound, they make you that ye shall neither be barren nor unfruitful in the knowledge of our Lord Jesus Christ.
[9] But he that lacketh these things is blind, and cannot see afar off, and hath forgotten that he was purged from his old sins.
[10] Wherefore the rather, brethren, give diligence to make your calling and election sure: for if ye do these things, ye shall never fall:
[11] For so an entrance shall be ministered unto you abundantly into the everlasting kingdom of our Lord and Saviour Jesus Christ.
Salvation is given at the point of a true born again experience. This precious gift then must be nurtured; failure to do so would be willful sin, and Salvation forfeited through negligence.