There are people mentioned in Hebrews 6:4-6 whom have “tasted” the good word of God and have been partakers of the Holy Spirit, only to fall away after that.
In the parable of the sowers, Jesus talked of two reasons why someone would fall away from the Lord after having known Him. One reason was because of the love of the world (the plant that grew up among thorns that choked it). The second example is the plant that grew up on rocky ground – sprouted up at first, but soon withered away. These are Christians who have lost the joy of the Lord.
Matthew 13:20-21
“The one on whom seed was sown on the rocky places, this is the man who hears the word and immediately receives it with joy; yet he has no firm root in himself, but is only temporary, and when affliction or persecution arises because of the word, immediately he falls away.”
There are multitudes, probably millions of people throughout history who have tragically fallen away from the Lord. We don’t know all the details about each one of these, but one thing we do know is that there was not one person among all those millions who had the joy of the Lord. Can you imagine someone ever falling away from Christianity whose main delight is in God Himself? It’s just not going to happen. One who keeps the joy of the Lord in the midst of tough trials is the one who is going to stick with God until the end. The one who rejoices in the Lord more than anything else in the world – more than money, more than spouses, and children and friends, and houses… the one who rejoices and delights in God Himself more than anything else is one of those few that has found the pearl of great price and sold everything to get it.
If you want to keep yourself from falling, rejoice in your Lord always. Always lay before the throne and look at His face with delight and hope.
Philippians 4:4 - “Rejoice in the Lord always; again I will say, rejoice!”