You will not be able to serve the Lord, &c. This was not said by way of discouraging them; but rather to make them more earnest and resolute, by setting before them the greatness of the undertaking, and the courage and constancy necessary to go through with it. (Challoner)

Josue knew the fickle temper of his subjects. He insinuates, therefore, that if they do not lay that aside, they will not stand to their engagements, (Calmet) and will irritate God the more, if they enter into a covenant with him, and afterwards prove inconsistent. Hebrew La thuclu, “you cannot,” may perhaps have the first u redundant; (Kennicott) as that is a letter which is often inserted or omitted at the transcriber’s pleasure. (Aben Ezra. Simon)

Hallet suggests that we ought to read lo thucelu, “you shall not cease,” which would obviate the apparent difficulty of Josue’s attempting, as it were, to cool the fervour of the people, by insinuating that they will not be able to stick to their resolutions, and that at a time when he is exerting every nerve to make them sensible of their duty, and to engage them to swear an inviolable fidelity to the Lord. “Cease not to serve the Lord, for he is a holy God, he is a jealous God, he will not forgive your rebellion, (Copssácos. Job xxxiv. 27,) nor your sins; if you forsake the Lord, and serve strange gods, then he will turn and consume you.” (Kennicott, Dis. 2.)

If we were to read with an interrogation, “Will you not be able? &c., it might answer the same end. Josue may be considered as starting an objection, which is but too common in the mouth of the slothful, and of many of the pretended reformers, Luther, &c., who endeavour to persuade the world that they are not able to comply with the rigour of God’s law, and even make his severity an encouragement for their despair. Josue replies that these pretexts are groundless, and that God, who has already done so much for them, (ver. 20,) will not abandon them in their wants, if they cry unto him; and that, instead of being dejected by the thought of his judgments, they ought to strive, with the utmost fervour, to comply with his divine will. (Haydock)

A general sometimes withholds the ardour of his soldiers, telling them that they are not a match for the enemy, in order to inflame their courage the more. (Menochius)

A torrent which has been long repressed, rushes forward with greater fury when the dam is broken down. (Haydock)