The Bible promises that "whoever believes in [Jesus Christ] should not perish but have everlasting life" (John 3:16) and says "by grace you have been saved through faith" (Eph. 2:8). But how much faith is necessary to obtain this salvation?
A person may wonder if he or she has believed enough to be saved. No wonder - there are those who claim that salvation is given only to those who have enough faith, a full faith, a special faith, etc., implying that one's faith in God's promise of salvation can be insufficient.
We know that faith can vary in degree or amount because of biblical testimonies. Jesus spoke of those with "little faith" (e.g., Matt. 6:30; 8:26; 14:31) and those with "great faith" (e.g., Matt. 8:10; 15:28). The man whose son was healed of an evil spirit had a small amount of faith, but asked for more (Mark 9:24). Jesus spoke of a faith as small as a mustard seed, the smallest agricultural seed known in those days (Matt. 17:20; Luke 17:6).
In the case of the man with the weak faith, it is clear that even the small amount he had was enough to see Jesus' miracle. Jesus Himself said that a mustard seed amount of faith was able to move a mountain or transplant a tree. This indicates that the amount of faith is not the issue; the issue is the object and the power that the faith accesses.
First, it should be noted that the Bible's promises of eternal life which mention faith or believing do not specify how much faith is necessary to obtain eternal life. (Acts 8:37 may look like an exception when Philip tells the Ethiopian eunuch that he can be baptized "If you believe with all your heart," but verse 37 is not in many important Greek manuscripts and is therefore omitted in many English translations. Even so, the eunuch's answer, "I believe that Jesus Christ is the Son of God" shows that the amount of faith is not the critical issue; rather, the issue is the object of faith.) Second, it is not actually our faith that saves us. When we look at Ephesians 2:8, "For by grace you have been saved through faith," we can differentiate between the cause (by grace) and the means (through faith). For example, if someone said that he washed his car with a hose, we understand that the hose was the means through which the cause of washing (water) was delivered. The hose did not actually wash the car, but the water did. Faith itself is not a force, but a channel through which God's power to save is delivered.
Third, the smallest amount of faith is sufficient for salvation if the object of that faith is worthy. Since it is God's grace that saves us in Jesus Christ, any amount of faith in Him will save. For example, someone who is drowning might have strong faith in a leaky boat that can not save her, or she could have weak faith in a seaworthy boat that can save her. The weakest faith is enough to save if it is in a worthy object. Some people have an enormous amount of faith in a false religion or in a false religious leader, but they will not be saved. The believer with the smallest amount of faith in the worthy Son of God will be saved, because salvation does not depend on the strength of one's faith, but on the sufficiency of Jesus Christ to do what He promises.
Examining the quality or amount of one's faith is a subjective exercise in futility. Salvation and the assurance of salvation only come by focusing faith on the worthiness of Jesus Christ and His promise to save. The issue is not how much we believe, but what we believe in. We either believe in Jesus Christ for eternal life, or we don't.