Joshua had just overwhelmed Amalek through the power of God (Exodus 17:11, 13), and God immediately sent a message to the young leader, which was to be memorialized for future remembrance. What is the message? "I will not rest or relent until Amalek has been obliterated!" God would continue to undertake, and victory is certain.
The Christian has been given the same message. God has sent His Spirit to war against your flesh (Galatians 5:17), and He will not rest or relent until every idol and fleshly indulgence has been utterly purged (Ezekiel 36:25). He will be your God! This promise delights the true believer because, as one brother said, he already sins more than he wants to.
As a side note - like Moses, we need to make it our business to remind one another of these promises.