Fighting in Saul's armour

“Then Saul dressed David in his own tunic. He put a coat of armour on him and a bronze helment on his head.” 1 Samuel 17:38
In 1 Samuel 17 we have the famous story of how David fought Goliath. The Philistines and the Israelites had drawn up to face each other in battle array. But before engaging, the Philistines offered Israel the chance to engage in representative combat: that is, to have one man from each side fight each other, and decide the battle based on that. Their representative was Goliath, a giant man, one of the Nephilim after the flood (see Gen. 6:4, Num. 13:33, 1 Chr. 20:4-8).
Israel’s obvious choice for a fighter to represent them was Saul. He was a head taller than everyone else (1 Sam. 9:23-24) and had won many battles (1 Sam. 11:11, 1 Sam. 15:7). Yet despite being Israel’s hero, he was unwilling to go and fight Goliath himself. Instead, he sat back and watched as for forty days Goliath came out and intimidated the Israelite army.
When David came to visit his brothers and saw this, he immediately volunteered to go and kill Goliath. Initially Saul tried to dissuade him, saying he was only a boy, but David responded by telling him how he had killed lions and bears who had attacked the sheep he was tending. Saul gave him permission – and then attempted to dress David in his own clothes and his own armour. But David was not a substitute for Saul. He had to do it his own way – without armour, without a sword: just with his shepherd’s sling and a stone from a stream.
Sometimes we will have situations where we’re like David. You’ll volunteer to do something, and somebody who could have done it but hasn’t, will then step in and try to tell you how to do it. Or, you might be the person who has neglected your responsibility, like Saul. The lesson for us is this: do what God has called you to do, in the way He has called you and gifted you to do it. Don’t try to fight in someone else’s armour, and don’t try to make someone else do something your way, if you’re not prepared to do it yourself. God has given each of us different abilities, and we are to use what we have in serving Him.