Isa 35:3-4, Jn 16:33, 1 Cor 9:18, 1 Cor 15:58, Eph 6:10, Heb 10:35
In the Old Testament, the prophet Oded told the Israelites to keep the word of God and not become discouraged. (2 Chronicles 15:7-8)
2 Chronicles 15:7 Be ye strong therefore, and let not your hands be weak: for your work shall be rewarded. 8 And when Asa heard these words, and the prophecy of Oded the prophet, he took courage, and put away the abominable idols out of all the land of Judah and Benjamin, and out of the cities which he had taken from mount Ephraim, and renewed the altar of the Lord, that was before the porch of the Lord.
In the Old Testament, Isaiah told the Israelites to be strong and not be afraid, because God will save them. (Isa 35:3-4)
Isa 35:3 Strengthen ye the weak hands, and confirm the feeble knees. 4 Say to them that are of a fearful heart, Be strong, fear not: behold, your God will come with vengeance, even God with a recompence; he will come and save you.
Jesus said we will have tribulation, but be courageous because He has overcome the world. (John 16:33)
John 16:33 These things I have spoken unto you, that in me ye might have peace. In the world ye shall have tribulation: but be of good cheer; I have overcome the world.
We must boldly preach the gospel. And we must not be shaken. Our efforts to spread the gospel will not be in vain. (1 Corinthians 15:58, Ephesians 6:10, Hebrews 10:35)
1 Corinthians 15:58 Therefore, my beloved brethren, be ye stedfast, unmoveable, always abounding in the work of the Lord, forasmuch as ye know that your labour is not in vain in the Lord.
Ephesians 6:10 Finally, my brethren, be strong in the Lord, and in the power of his might.
Hebrews 10:35 Cast not away therefore your confidence, which hath great recompence of reward.