Great stories have sequels.
1 Samuel 7 is the sequel to an earlier story in the book of Samuel. In 1 Samuel 4, the people of Israel went to war with the Philistines and lost. Instead of turning to God and asking why he was not with them, they tried to manipulate God. They took the ark of God out to battle thinking this would force God to act on their behalf. God will have to defend us since we have his ark, right?
They lost the battle and they lost the ark.
Manipulation led to misery.
Now turn to 1 Samuel 7 which is the sequel to that story. This time the people of Israel are forced into battle with the Philistines again. This time, however, they do not rely on manipulation and religious formality. Instead, they rely on repentance and prayer.
The people of Israel go to the prophet Samuel and repent of their sins and ask Samuel to pray on their behalf. They turn from the idols of their culture and embrace the living God.
Consequently, when they go to battle with the Philistines the Lord gives them the victory. The people of Israel find the mercy of God through repentance and prayer.
That same principle applies today. The mercy of God is not found in trying to manipulate God, trying to be a better person, or trying to earn God's favor. Rather, you find the mercy of God when you turn from sin to the Triune God. You find the mercy of God when you express your faith through prayer.
The God who demonstrated mercy to Israel over 3,000 years ago is the same God today. Even after Israel rejected God for 20 years, God remained patient with them and was merciful. For you, even if you have rejected God for weeks or years, God is still patient with you and will be merciful to you if you repent and pray.