If they will confess their sins and the sins of their ancestors

 Leviticus 26-27

Do not make idols or set up an image or a sacred stone for yourselves, and do not place a carved stone in your land to bow down before it. I am the Lord your God.

Observe my Sabbaths and have reverence for my sanctuary. I am the Lord.

If you follow my decrees and are careful to obey my commands, I will send you rain in its season, and the ground will yield its crops and the trees their fruit. Your threshing will continue until grape harvest and the grape harvest will continue until planting, and you will eat all the food you want and live in safety in your land. I will grant peace in the land, and you will lie down and no one will make you afraid.

I will look on you with favor and make you fruitful and increase your numbers, and I will keep my covenant with you. You will still be eating last year’s harvest when you will have to move it out to make room for the new.

But if you will not listen to me and carry out all these commands, and if you reject my decrees and abhor my laws and fail to carry out all my commands and so violate my covenant, I will bring on you sudden terror, wasting diseases and fever that will destroy your sight and sap your strength. You will plant seed in vain, because your enemies will eat it. I will set my face against you so that you will be defeated by your enemies; those who hate you will rule over you, and you will flee even when no one is pursuing you.

But if they will confess their sins and the sins of their ancestors—their unfaithfulness and their hostility toward me, which made me hostile toward them so that I sent them into the land of their enemies—then when their uncircumcised hearts are humbled and they pay for their sin, I will remember my covenant with Jacob and my covenant with Isaac and my covenant with Abraham, and I will remember the land.

These are the decrees, the laws and the regulations that the Lord established at Mount Sinai between himself and the Israelites through Moses.

Why?

The Lord already knows that the Israelites will not listen to the word of the Lord, their hearts are attracted by idols, and they are unfaithful to the Lord.

But the Lord is full of love and mercy to his people, if they confess their sins and the sins of their ancestors and return to the Lord and humble themselves, the Lord will remember the covenant with Abranhsm, and Jacob and Isaac. 

Why?

No matter what happened, if anyone confesses his sins and returns to the Lord, there is a way to get salvation from the Lord. For the Lord is full of love, and everyone was a sinner.

It is no my effort or my work, but it is the love and mercy of the Lord come upon me, so I can be a Christian, a son of the Lord. Not because of who I am, but because of who He is, not because of what I have done, but because of God's salvation and great works on me.

How?

Praise the Homy name of the Lord, and give thanks to the Lord, obey God's words forever!