Those who truly understand the purpose of water immersion according to the scripture recognize that it is the means of attaining to the blood of our Savior through the likeness of His death, burial, and resurrection. It is where we share His impalement and meet His blood. Without water immersion, we cannot "crucify" (impale on an upright post) nor "bury" the old man of sin, nor can we rise up from the grave, as He did, to walk (be born again) in His likeness (Rhomaios [Romans] 6:3-5).
There is no part or place in scripture that ever says or even vaguely hints that water immersion is an "outward sign of inward faith." If it was, then the purpose of water immersion would be an external event to show the world that we are in obedience instead of the internal event of "an answer of a good conscience before Yahuwah" (1 Keph [Peter] 3:21). Besides, we need show no one except Yahuwah that we intend to obey Him. It is really nobody else's business.
The same Protestant people of whom we spoke above would be shocked to learn that there is an outward sign of inward faith to be found in the scripture. It is found in the very same passages they attempt to use to show that faith alone saves (without the so-called works of repentance and immersion).
Consider this. Our Master said, "Whoever therefore shall confess me before men, him will I confess also before my Father which is in the sky." (MattithYahuw [Matthew] 10:32) This confession that "whoever" is making is that Yahushua is the only begotten Son of our Eloah. Those that confess Him in this manner are stating that they have accepted Him as such and in so doing, after having repented and been immersed, have become sons of our Eloah (1 Yahuchanan [John] 3:2).
So the "outward sign of inward faith" is not the act of immersion, which is an internal event between the repentant believer and Yahuwah, but the outward event of publicly acknowledging Yahushua as the Son of our Eloah Yahuwah (Rhomaios 10:9-10).