The phrase “be sure your sin will find you out” is fairly recognizable. We take it to mean that when we sin, we cannot remain hidden – it will be found out. The results of the sin, the consequences, or God Himself, will see to it that we cannot just continue sinning. And this is indeed true; “for whom The Lord loveth he chasteneth, and scourgeth every son whom He receiveth” [Heb 12:6] and, “whosoever is born of God doth not continue to commit sin; for His seed continues to remain in him: and he cannot continue to sin, because he is born of God.” [1 Jn 3:9]
However, look at the context of the phrase in Numbers,
"And Moses said unto them, if ye will do this thing, if ye will go armed before The LORD to war, and will go all of you armed over Jordan before The LORD, until He hath driven out His enemies from before Him, and the land be subdued before The LORD: then afterward ye shall return, and be guiltless before The LORD, and before Israel; and this land shall be your possession before The LORD. But if ye will not do so, behold, ye have sinned against The LORD: and be sure your sin will find you out." [Num 32:20-23]
The specific sin that Moses lists is not fighting for the land! If the Israelites became content and refused to fight, if perhaps they lost the zeal to fight, if they lacked faith in God’s promise to give the land – their sin would find them out! The word for “find you out” means to “reach; to happen; to befall.” If the Israelites did not fulfil their task by God to fight for the land He promised them, that sin, that lack of faith, would befall them – it would come down upon them and they would be punished by God for their disobedience.
In the same way, we must “fight the good fight of faith” [1 Tim 6:12]. We must “put on the whole armour of God” [Eph 6:11] and fight for the “land” that Jesus has given us. We have victory through Jesus: we can overcome the giants of sin, we can take spiritual land and be purer for our Bridegroom.
"But thanks be to God, which giveth us the victory through our Lord Jesus Christ." [1 Cor 15:57] "The horse is prepared against the day of battle: but victory is of The LORD." [Prov 21:31] "For whatsoever is born of God overcometh the world: and this is the victory that overcometh the world, even our faith. Who is he that overcometh the world, but he that believeth that Jesus is the Son of God?" [1 Jn 5:4-5]
Do not become apathetic or content with where you are in your walk with Christ. Do not faint at the tribulation of this world, but rejoice in Jesus and press forward. “Endure affliction, as a good soldier of Jesus Christ” [2 Tim 2:3] and “press toward the mark for the prize of the high calling of God in Christ Jesus.” [Phil 3:14]