I think the term 'backslider' can cover people who are there for many different reasons, much as depression covers a condition that people have but each one has it because of a different set of reasons.
Jesus in the parable of the sower (or was it seeds) and how different seeds have different outcomes. Some hear the word and grow up fast, but are soon choked by worries of the world and wither and die. Others are abundantly fruitful, others still fall on rocky soil and die through lack of moisture. In others words the word takes root in three of these cases, but through lack of nurture or foundation in the life with God, or through letting the things of life distract one from the life with God, one's faith dies. Some never accept the word from the start and never make that decision to enter life with God. And then some guard their precious life with God and it grows and thrives (Luke 8)
Elsewhere, people are told how to handle people who do not listen to correction or rebuke when they wrong someone, and if they continue to resist, then they are to be treated as an outsider, or a non-christian, one who does not know what it really means to be a Christian (Matthew 18:15-17).
Other times, the church is exhorted to rescue some from the fire (Jude 22-23).
I think people should take each person on a case by case basis - it's insensitive to simply say that some people who have backslidden were never Christians to begin with or that they lacked faith. Is that not a judgement? Is God not the only one who can see people's hearts and have the right to judge? You don't know whether they were hurt by others in the church, or really felt betrayed by God and struggle with that. Perhaps the church failed them and bore them a bad witness on God's behalf.
I know this because I'm not going to church right now. I've tried for several years after being hurt by several people over several churches. I continue not to fit in, and continue to be unhelped. It has become painful to go to church because it reminds me that I was hurt there and that I've become a person that feels alienated from the things I still believe in. I need a break from the church, it's become poisonous to me.
Still, I believe what I've always believed as a Christian. If some dismiss me as backslidden and never saved to begin with, that's their judgement on me, and they're wrong. If someone sees me as one in danger and relates to me based on who I am in Christ and not what I appear to be now, then I think that's actually more Christlike. While I am responsible for my life, there is no doubt the Christian life is to be lived as one in community, and if the church is no longer that, then I don't want to be a part of it.
I say that the majority of backsliders know everything you know as a Christian, but it sounds old and useless and irrelevant and trite. Come to them with a respect for what they know. But move on to what they still believe and bring it back to them - if you believe in God, are you spending time with God daily with His word? In Prayer? Are you counting your blessings? Helping others? Sometimes they need to be helped along like a friend helps a wounded person until they can walk again, even though they kick and scream from the pain. Never give up on a backslider.