Are rewards eternally lost when they are lost? Or are you eligible to regain those rewards if you repent, return, and persevere correctly for that reward after having fallen away?
Thanks for the question. Here are some things to consider.
First, once you lay up treasure in heaven (Matt 6:19-21), it seems to be permanent. There is nothing to suggest that treasure can be lost. And if it can’t be lost, then it can’t be regained.
Second, you can lose time and opportunity to serve the Lord to gain eternal rewards. If you waste ten or twenty years of your life backsliding or doing unrewardable things, you don’t get that time back to lay up treasure in heaven. Those works will be burned up (1 Cor 3:11-15). In other words, once the opportunity is gone, it’s gone forever and cannot be regained. But so long as you are alive, you can have more opportunities to lay up more treasure in heaven.
Third, you can lose opportunities to gain reward through the people you have influenced. If you disciple someone, and he or she is fruitful for Christ, you can be rewarded for what that person does. However, if that person falls away, you lose those opportunities to earn rewards. That is suggested by what John said in his second epistle: “Look to yourselves, that we do not lose those things we worked for, but that we may receive a full reward” (2 John 8). John was urging his readers to be conscientious, so he would receive a full reward. Once those opportunities are gone, they cannot be regained. But if that person comes back to the faith, he will have new opportunities to earn rewards.
Fourth, there are some overcomer rewards—ruling with Christ, eating from the tree of life, wearing overcomer garments—that depend on persevering in faith to the end of your life. Since you are not given those rewards in this life, you can’t lose them in this life or regain them. At best, you can be on a trajectory of winning them. However, if you die denying Christ, you will lose out on receiving those rewards.
So, can eternal rewards be eternally lost and regained? No. You can gain them. You can miss out on them. But, so far as we know, you cannot lose and regain them.