am assuming the Christian faith and referencing the Bible and the God of the Bible: Abraham believed God and it was counted unto righteousness. By faith Abraham obeyed God. (Romans 4:3; Heb. 11:8).
You don’t just believe He exits for the demons do that. And faith without works is dead: true faith is manifested and proved by good works—you are known and rewarded by your fruit. Faith is authenticated by deeds! You believe Him and take Him at His Word, trusting Him with your heart and life. You must be willing to do His will and obey His commands; surrender to the Lordship or ownership of your life. Is Christ your Lord? Do you unashamedly confess His name and stand up for Him?
True faith entails love in the heart for we believe not just in our minds as much as in our hearts. (cf. Romans 10:9–10). Do you believe the God who is and that He is with you? You don’t have to repeat a creed every day religiously but grow in your faith by obedience. In short, trust and obey! That is the only true measure of faith. It isn’t how much you believe but how well you obey. Remember, it’s not the amount of faith but the object that matters. It must be settled solidly in Christ alone.
Also, true faith is connected with true repentance—they go hand in hand and it may not be how much you believe but how thorough your repentance is. You must grow in your faith and develop a relationship with Jesus by confession of known sin and walking in the Spirit. We learn to walk by faith, not by sight (cf. 2 Cor. 5:7). This means you need to be filled with the Spirit continually so you do not walk in the flesh and sin. This also means producing the fruit of the Spirit: love, joy, peace…. Thus, it’s not just having a sort of faith, but having saving faith that translates your creeds into deeds!