Today I am going to focus on a portion of the story of David and Goliath which is found in 1 Samuel 17:29,45-46,49. From what I remember of looking up how tall Goliath was, he was at least 9 feet tall. David on the other hand was not old enough to be in the army at the time.
After having a conversation with King Saul, David was appointed to be the champion of Israel and went out to meet Goliath. David believed the Israelites had a just cause and because of that the Lord was with them and they would be victorious.
Here are some of David's words to Goliath: "Thou comest to me with a sword, and with a spear, and with a shield: but I come to thee in the name of the Lord of hosts, the God of the armies of Israel, whom thou hast defied. This day will the Lord deliver thee into mine hand; and I will smite thee, and take thine head from thee."
As I read and understand these words it seems to me that David had no doubt he was going to defeat Goliath. His experience and confidence with the use of a sling combined with his faith in God such that he believed he could not lose. It would not surprise me if no thought of loss crossed his mind, only the knowledge that he would defeat Goliath because he was engaged in a just cause in the service of the Lord.
And with the first stone David let fly from his sling, he felled Goliath, and defeated the champion of the Philistines.
From this story it appears it was relatively simple and easy for David. What we must remember is that anything we do that is aligned with the will of God will turn out as God wants it to. That may mean that we decide to do something and in a day it is done or it may mean that it takes us decades to complete. But a simple way to think of it is if we are engaged in a just cause in the service of the Lord then whatever that cause is will be successful.