Was there any reason for the Israelites to doubt? No, of course not. The Lord had delivered them from the Egyptians with a mighty hand, and they had witnessed numerous miracles, all of which were made possible by Him. They should have known better than to disbelieve and doubt the all-powerful God who was with them. He delighted in them and would have fulfilled His promise to them in Moses' day--had they not rebelled.
What the Israelites failed to remember is that with God on their side, it didn't matter what enemy they faced. Physical size and human strength is irrelevant is comparison with Almighty God. When the Lord is on your side, it matters not who is against you, for you are on the winning side, even though you will typically not be in the majority.
Consider some examples from Scripture where the minority won--because they were on God's side! During the years in building the ark, Noah was very much in the minority, but he won (Gen. 6-8)! When Joseph was sold by his brethren into Egypt, he was in a decided minority, but he won (Gen. 37-45)! When Moses appeared before Pharaoh and demanded the freedom of the Israelites, he too was very much in the minority, but he won (Exo. 5-15)! When Gideon and his three hundred followers with their pitchers and lamps put the Midianites to flight, they were an insignificant minority, but they won (Judg. 7)! When Samson crushed the temple and destroyed his enemies, he was very much in the minority, but he won (Judg. 16)! When the young man David went out to meet the giant Goliath in battle, he was, in size, the minority, but he won (I Sam. 17)! When Elijah brought down fire from heaven and put the prophets of Baal to shame, he was a notable minority, but he won (I Kin. 18)! When Jesus Christ was crucified between two thieves by the Jews and Romans, he was a conspicuous minority, but He won (Matt. 27 & 28)!
Friends, I hope it is plain to see that size is irrelevant if you're on the Lord's side. Numbers are irrelevant if you're on the Lord's side. It doesn't matter who is against you. It doesn't matter what the odds of success are. If you are on the Lord's side, you will be victorious! I Corinthians 15:57 says - "Thanks be to God, who gives us the victory through our Lord Jesus Christ."
Many times we will find ourselves in the minority when it comes to our faith. It may be at the office, in the shop, at school, or even in the home. May we take strength and courage from those who found themselves in the minority and won! Let us, like them, be found in God's service doing His work, regardless of the foes that rise against us. May we fight the good fight that the Lord has laid out for us, and may we not grow weary in doing good. I Corinthians 15:58 says - "Therefore, my beloved brethren, be steadfast, immovable, always abounding in the work of the Lord, knowing that your labor is not in vain in the Lord." There is much to be done in the Lord's kingdom, but it will go undone by those who doubt God and view themselves as small and powerless--like grasshoppers.