Repentance and faith for the forgiveness of sin. This is not a New Testament idea, but stretches back to the Old Testaments as well. 2 Chronicles 6 repeatedly tells the people that if they sin, they must turn back to God (repent), ask for His forgiveness, and look to Him for salvation. We have a fuller revelation of this salvation in the person and atoning work of Jesus Christ, but the method has really not changed.
If your people Israel are defeated before the enemy because they have sinned against you, and they turn again and acknowledge your name and pray and plead with you in this house, then hear from heaven and forgive the sin of your people Israel and bring them again to the land that you gave to them and to their fathers (2 Chronicles 6:24-25).
When heaven is shut up and there is no rain because they have sinned against you, if they pray toward this place and acknowledge your name and turn from their sin, when you afflict them, then hear in heaven and forgive the sin of your servants, your people Israel (2 Chronicles 6:26-27).
If they sin against you–for there is no one who does not sin–and you are angry with them and give them to an enemy, so that they are carried away captive to a land far or near, yet if they turn their heart in the land to which they have been carried captive, and repent and plead with you in the land of their captivity, saying, ‘We have sinned and have acted perversely and wickedly,’ if they repent with all their mind and with all their heart in the land of their captivity to which they were carried captive, and pray toward their land, which you gave to their fathers, the city that you have chosen and the house that I have built for your name, then hear from heaven your dwelling place their prayer and their pleas, and maintain their cause and forgive your people who have sinned against you (2 Chronicles 6:36-39).