Look to yourselves, that we lose not those things which we have wrought, but that we receive a full reward. -2 John 1:8
John points out the importance of continuing in the faith, following the commands to love one another and remaining strong in the doctrines of the faith so as to guard against being deceived.
If we don't stay strong in these areas then we lose the growth in Christian sanctification that we had attained. It's like a muscle.
So in the Christian life if you don't exercise your spiritual muscles you will lose them and much of what you have done will be burned up as wood hay and stubble when you stand before God at the end.