Our faith in God is essentially our confidence and trust in Him. When we get to know who God really is and understand His love for us we respond by putting our trust in Him. The immediate change that comes in us is to believe whatever He has said and want to do everything He wants us to do.
The Bible tells us that 'faith' comes by hearing the words concerning Christ (Rom.10:17). It is when we hear about God sending His Son to die in our place because of His love for us that we see ourselves as sinners who deserve punishment for our sins and turn to God for forgiveness. This is 'repentance' and 'faith' seen side by side. Without repentance there cannot be a true faith in God and without faith we cannot receive what He is offering us for our salvation. When some people associate faith with only 'believing that Jesus died for sins and rose from the dead' without any repentance taking place and a trust building up in God, they miss the real point. Such people are not really saved!
One of the fruit of a living faith in God is our desire to obey Him. Without such obedience our so-called faith is bogus (Jas.2:26). But here comes a lot of confusion because of an exaggerated preaching of grace that says that obedience pertains to living under the Law, and since a Christian has been freed from the Law there should not be any 'legalistic' emphasis on obedience! But this leads to carelessness with sin as if how we actually live doesn't matter because we are 'under grace' (turning grace into a license for sin Jude.4;Ro.6:15).
What we see in practical life is that when we want to obey God we immediately come across resistance from our flesh, fed by the attraction of the world and the schemes of the devil. That is when we have to deny ourselves, put our sinful desires to death on the cross and follow the life of Jesus to do God's will (Lk.9:23). That is what our faith in God and His word naturally leads to. It takes strong determination and effort from our side to obey God at all times. This is why the Christian life is called a battle; it's not about having a good time singing songs and going out for picnics together!
Paul understood that as an apostle his task was to bring about obedience to God through faith (Ro.1:5). He worked day and night to share with the people whatever was necessary to help his hearers to come to maturity as Christians (Co.1:28). This was a big challenge, but he didn't spare any effort to convey the full measure of the purpose of God to his hearers (Ac.20:20,27).
The Holy Spirit warns us that some will fall away from the (true) faith, getting their consciences seared - not allowing the Spirit to convict them about what is wrong with their practical lives after professing a faith in Christ (1Ti.3:1,2). Deceitful spirits are at work telling people that when they are under grace they are 'safe' no matter what they do.
If we truly love God let's show our faith by obeying Him in everything (Jn.14:15).