The pre-battle speech of the כהן משיח begins with the phrase: שמע ישראל אתם קרבים היום למלחמה (20:3). The Gemara in Sotah (42a) relates, “Even if you only have the merit of fulfilling the commandment of the reading of שמע alone, you are worthy that Hashem should save you”
Another דרשה taught in Gemara Sotah (44a) based on a phrase found only a few verses later, seems, however, to contradict the above. The שוטרים make the following announcement:מי האיש הירא ורך הלבב ילך וישב לביתו “What man is fearful and faint-hearted? Let him go and return to his house.” ר' יוסף הגלילי interprets the phrase הירא ורך הלבב, as follows: הירא מעברות שבידו “This refers to a person who’s fearful of the transgression in his hand.” This is an individual who was encouraged to return home from battle.
The דרשה of ר' יוסף states explicitly that a person should return home from battle if he is fearful of any possible transgressions he may have violated, yet previously the Gemara stated that Hashem assures salvation with the fulfillment of merely the recitation of the ?שמע
R’ Yechiel Reuven Mandelbaum in his Sefer דודאי ראובן answers this question by first posing two additional difficulties. Firstly, why does ר' יוסף use the phrase הירא מעברות שבידו, the word שבידו is superfluous, הירא מעברות שלו or simply הירא מעברות would have been sufficient; why must ר' יוסף mention “The transgressions that are in his hands? Secondly, why does ר' יוסף see in this Pasuk a requirement for all those who are fearful of violating transgressions to return home? If an individual is careful to follow the charge of the כהן משיח and recite שמע with the proper כוונה then he has accepted עול מלכות שמים and has therefore achieved תשובה.
R’ Mandlebaum explains that one of the requirements of תשובה is עזיבת החטא. Although one has accepted עול מלכות שמים if a person’s יצר הרע is too over-powering and he cannot remove himself from a particular sin, he has not accomplished תשובה. It is the individual that recites שמע with the purest of intent but cannot remove himself from sin and retains that sin בידו that ר' יוסי orders to return home. The term בידו indicates that this individual has not performed תשובה since he has failed in the challenge of עזיבת חטא. As a result his שמע cannot possibly be pure and its recitation is compared to one who immerses in the מקוה with a שרץ in his hand.
In conclusion, there is no contradiction between the charge of the כהן משיח and the שוטרים. The שוטרים come to clarify that of one has not performed תשובה completely then his שמע will actually not save him. Therefore he should preferably return home.