So, now we have the Gospel, the mystery of the union of God with His people, which is truly good news. And we have the requirement that in order to be part of God's people we need to repent. Included in that "repent" message from the lips of Jesus was "Repent and believe in the gospel." (Mark 1:15) It is abundantly clear in God's Word that faith is another necessary prerequisite to being part of salvation and the subsequent union with God. It would seem to be a really good idea, then, to figure out just what this "faith" thing is, because not everyone is clear on it. What do we learn about faith from the pages of Scripture?
Well, we know that "without faith it is impossible to please Him, for whoever would draw near to God must believe that He exists and that He rewards those who seek Him." (Heb 11:6) Faith, then, is absolutely necessary. But Hebrews 11 tells us more. We know quite clearly that "faith is the assurance of things hoped for, the conviction of things not seen." (Heb 11:1)
First, there is the word "faith". It means "to be convinced (by evidence or argument)." Faith in God, then, would entail being convinced that God is and that He is reliable, faithful, truthful. Faith in God is simply agreeing with Him about who He is. Note that this isn't in contradiction to reason or evidence; it is in agreement with both. And let's not miss out on those two factors in the text -- assurance and conviction. The word for the first, "assurance", is that which supports, the essence of something. The word for the second is conviction, evidence, or proof. Genuine, biblical faith is faith that agrees with God regarding His existence and who He is. It is assurance that He will do what He promises ("rewards those who seek Him") and the proof of things we don't yet see. It is premised on God, so it is not "credulity", the blind belief in something without evidence or reason. It simply takes the evidence and reasoning to its clear conclusion. God exists and is who He says He is, so He will surely do as He has said He will.
This presupposes something. It assumes that God exists and that we know what He has promised. Not all, necessarily, but there must be some known truth claims and promises. That is, genuine biblical faith requires first and foremost that we believe the truth. It is not biblical faith to believe that which is not true.
In John's Gospel he often uses the phrase "believe in" or "believe on". Most literally John is using a phrase that is most accurately translated to "believe into". That is, biblical faith is an investment. It is a "leaning on", so to speak, that which is believed. Habakkuk wrote, "The righteous shall live by his faith." (Hab 2:4) This is, apparently, a really important concept because Paul repeated it in Rom 1:17 and in Gal 3:11 and the author of Hebrews also included it in Heb 10:38. Notice, then, the key component here. Faith is lived. That puts it beyond mere "mental acquiescence." It is a principle of living. James informed his readers that faith without works was dead (James 2:17). Indeed, "You believe that God is one; you do well. Even the demons believe and shudder!" (James 2:19) Mere mental agreement with facts is not saving biblical faith. That only rises to the level of demons and is classified as dead. Living, saving faith produces changes in behavior because living, saving faith is a placing of confidence in that which is believed. We will always act on genuine faith. Real faith is visible (Matt 9:1). Like repentance, genuine faith produces corresponding action. (Not vice versa. Our actions do not produce faith.)
In his argument that we are saved by faith apart from works, Paul uses Abraham as his "proof text" (Romans 4). He starts with the claim that "Abraham believed God, and it was counted to him as righteousness." (Rom 4:3) That is biblical, saving faith. Notice that Abraham's faith was "counted to him as righteousness." That is, Abraham's righteousness was attributed to him on the basis of his faith (not his works).
Where does faith come from?
We already saw that repentance was granted by God (Acts 11:18; 2 Tim 2:25). The Bible tells us that faith is also a gift. Paul wrote that all believers have a "measure of faith that God has assigned." (Rom 12:3) He told the Philippians that they were "granted, on behalf of Christ, not only to believe in Him, but also on behalf of Him to suffer." (Phil 1:29) Jesus explained to His disciples that the reason people don't believe was because "no one can come to Me unless it is granted him by the Father." (John 6:64-65) So faith is a gift, a "grant", from God that we, then, exercise. And it isn't necessarily stagnant. Faith can grow (2 Thess 1:3). It can be increased by hearing the Word of God (Rom 10:17). It can be increased by prayer (Mark 9:24; Luke 17:5).
Faith is essential to salvation. We must believe in Christ for salvation. That is, we must agree that He is our Savior and trust Him to save us. In the process, we must repent (turn from sin) and believe (turn toward Him and place our confidence in Him). This faith is more than mere belief -- intellectual assent. It is an investment of confidence in the existence and nature of God that will necessarily produce a change in the behavior of the believer. It is a lifestyle. "The righteous shall live by his faith." Without it, no one can please God. On the other hand, "Believe in the Lord Jesus Christ and you shall be saved." (Acts 16:31)