1 Timothy 6:12 (NKJV)
To be successful in doing something, you have to do it the right way. If someone wants to play basketball, he can't play by the rules of football and succeed. In fact, that person would have all kinds of problems trying to play one sport by the rules of another. If someone wants to be a better swimmer, but they spend all their time learning about racing cars, and they never get into the pool, that person will never succeed in swimming. To succeed, to win, the right methods are necessary in whatever we do.
Whether they know it or not, Christians are called to fight. But what and how do we fight? The Bible tells us in 1 Peter 5:8-9 that we have an enemy, the devil, who walks around like a roaring lion looking for someone to devour. Peter says that we must resist the devil, which literally means "to stand against", or "to oppose". We have an enemy, so what do we do? Peter gives us the same answer that Paul gave; we resist, we stand against the devil, in faith, this is the fight of faith.
But, how do we resist the devil and fight in faith? We fight the faith fight by taking a stand on God's Word. We hold on to the truths about what God has done for us through Jesus, and when the devil comes against us, we speak, and act on, God's Word; that is how we exercise our faith and fight the faith fight. Jesus has already defeated the devil, but we must resist the attacks the devil makes against us, through faith in God and His Word, this is the fight of faith.