Gerhard provides a lovely illustration of the truth that weak faith is still true faith. These are two different distinctions: true vs. false faith; weak vs. strong (or heroic) faith. True/false faith is a distinction of object (Christ or not). Weak/strong faith is a distinction of degree. One might also say that an apple tree is an apple tree whether it yields one or a thousand bushels of apples; whether it fruits every year or only some years.
Mediocre believers, provided they truly believe, are believers too, and weak faith is faith too because it apprehends the same merit of Christ as does a firm and strong faith. Just so, a little boy and a giant might grab the same gold ring even though they do not hold it with the same strength. Therefore a distinction must be made between the object of faith and its degree. With respect to its object the strong and the weak have one and the same faith. With respect to degree there is a distinction between strong and weak faith, but this distinction does not at all remove from weak faith its truth and effect of justification. For this reason Christ declares absolutely without any respect for strong or weak faith (John 5:24): “The one who believes in Him who sent Me has eternal life and does not come into judgment because he has passed from death to life.” Also, Peter declares about all true believers (1 Pet. 1:9) that the end of their faith “is eternal life.”
On Death, § 185(5).