Genesis 19:15-17 KJV
 And when the morning arose, then the angels hastened Lot, saying, Arise, take thy wife, and thy two daughters, which are here; lest thou be consumed in the iniquity of the city.  And while he lingered, the men laid hold upon his hand, and upon the hand of his wife, and upon the hand of his two daughters; the Lord being merciful unto him: and they brought him forth, and set him without the city.  And it came to pass, when they had brought them forth abroad, that he said, Escape for thy life; look not behind thee, neither stay thou in all the plain; escape to the mountain, lest thou be consumed.
Genesis 19:24-26 KJV
 Then the Lord rained upon Sodom and upon Gomorrah brimstone and fire from the Lord out of heaven;  And he overthrew those cities, and all the plain, and all the inhabitants of the cities, and that which grew upon the ground.  But his wife looked back from behind him, and she became a pillar of salt.
Likewise—but centuries later—an invitation to be saved that included a lifestyle change was effectively rejected at the heart level:
Luke 9:59-62 KJV
 And he said unto another, Follow me. But he said, Lord, suffer me first to go and bury my father.
 Jesus said unto him, Let the dead bury their dead: but go thou and preach the kingdom of God.
 And another also said, Lord, I will follow thee; but let me first go bid them farewell, which are at home at my house.
 And Jesus said unto him, No man, having put his hand to the plough, and looking back, is fit for the kingdom of God.
To backslide is to turn back to your old ways of living, to your old self (even in the least), to the old man. It is the same as looking back, as did Lot’s wife. It is the same as giving priority to anything over God. That could mean doing something seemingly as harmless as visiting or bidding farewell to loved ones, or even attending to the dead first, before following Christ, when He has directed you to do something else first.
To look back is loving the world, loving what you were or loving what you have or others more than God. It means putting others, things, and self pleasure, and being a men pleaser over, before, and above pleasing God. It means doing anything, rather than obeying God. Not walking in God’s ways in the least in order to do your own thing instead is disobedience. Let that sink in.
Here’s how Jesus described it:
Luke 14:26-33 KJV
 If any man come to me, and hate not his father, and mother, and wife, and children, and brethren, and sisters, yea, and his own life also, he cannot be my disciple.  And whosoever doth not bear his cross, and come after me, cannot be my disciple.
[Remember, sit down first and count the cost of accepting God’s gift of salvation and your surrender to God. Consider what all it is and what it all entails.]
 For which of you, intending to build a tower, sitteth not down first, and counteth the cost, whether he have sufficient to finish it?
 So likewise, whosoever he be of you that forsaketh not all that he hath, he cannot be my disciple.
Let that sink in.