Above all, you must live as citizens of heaven, conducting yourselves in a manner worthy of the Good News about Christ. Then, whether I come and see you again or only hear about you, I will know that you are standing together with one spirit and one purpose, fighting together for the faith, which is the Good News.
— Philippians 1:27
Stop and think for a moment about people who backslide spiritually. It doesn't happen overnight. You don't call up your mature Christian friend and say, "Hey, dude. Want to backslide? Seriously? Pick you up at 7:00."
There might be some foolish person out there who has done that, but generally this isn't the case. Backsliding usually starts with tolerating something that is sinful, something you never would have gone near before, but now you accept. You sort of put up with it and maybe even play around with it, thinking, Oh, it isn't going to hurt me. I am so strong. I have walked with the Lord all of these years. I have memorized so many Bible verses.
Don't think you can't still fall.
Have you ever seen a baby rattlesnake? They have their little fangs and their little tails and their little rattles. It could actually look cute if you are into that sort of thing. But then it bites you. And the venom of a baby rattlesnake is more potent than that of an adult rattlesnake.
In the same way, backsliding starts with accommodating. You sort of turn a blind eye to something. Then you give in to it and legitimize it. And then you start making excuses for it and defending it. The next thing you know, you embrace it as a normal lifestyle. But that is not the behavior of a real saint, of a real follower of Jesus.
If you want to finish well in the race of life and live joyfully, if you want God to complete the work He has begun in you, then you need to press on as a follower of Christ. He will give you the strength to do it. But you need to ask for that strength every day.