At the commencement of a 2520 punishment, God himself always announces its beginning in person or through the voice of His prophets.  For Israel and Judah this voice was Isaiah.

Isaiah 1:2 Hear, O heavens, and give ear, O earth: for the LORD hath spoken, I have nourished and brought up children, and they have rebelled against me.
3 The ox knoweth his owner, and the ass his master's crib: but Israel doth not know, my people doth not consider.
4 Ah sinful nation, a people laden with iniquity, a seed of evildoers, children that are corrupters: they have forsaken the LORD, they have provoked the Holy One of Israel unto anger, they are gone away backward.
5 Why should ye be stricken any more? ye will revolt more and more: the whole head is sick, and the whole heart faint.
6 From the sole of the foot even unto the head there is no soundness in it; but wounds, and bruises, and putrifying sores: they have not been closed, neither bound up, neither mollified with ointment.
7 Your country is desolate, your cities are burned with fire: your land, strangers devour it in your presence, and it is desolate, as overthrown by strangers.
8 And the daughter of Zion is left as a cottage in a vineyard, as a lodge in a garden of cucumbers, as a besieged city.
9 Except the LORD of hosts had left unto us a very small remnant, we should have been as Sodom, and we should have been like unto Gomorrah.

God saw that both Israel and Judah's cup of iniquity was full to overflowing; in His eyes they had become as Sodom and Gomorrah.

Isaiah 1:10 Hear the word of the LORD, ye rulers of Sodom; give ear unto the law of our God, ye people of Gomorrah. 11 To what purpose is the multitude of your sacrifices unto me? saith the LORD: I am full of the burnt offerings of rams, and the fat of fed beasts; and I delight not in the blood of bullocks, or of lambs, or of he goats. 12 When ye come to appear before me, who hath required this at your hand, to tread my courts? 13 Bring no more vain oblations; incense is an abomination unto me; the new moons and sabbaths, the calling of assemblies, I cannot away with; it is iniquity, even the solemn meeting. 14 Your new moons and your appointed feasts my soul hateth: they are a trouble unto me; I am weary to bear them. 15 And when ye spread forth your hands, I will hide mine eyes from you: yea, when ye make many prayers, I will not hear: your hands are full of blood.

Just like many do in this day and age, the Israelite nations were justifying themselves by asking God to forgive them of their sins through the sacrificial system but they had no interest in forsaking their sins and obeying His commandments.  Justification without sanctification is an abomination to the Lord, or in simple terms: saying you're sorry for a wrong you have committed but in your heart you have no intention of giving up this wrong, your "sorry" is a lie!

Isaiah 1:16 Wash you, make you clean; put away the evil of your doings from before mine eyes; cease to do evil;
17 Learn to do well; seek judgment, relieve the oppressed, judge the fatherless, plead for the widow.
18 Come now, and let us reason together, saith the LORD: though your sins be as scarlet, they shall be as white as snow; though they be red like crimson, they shall be as wool.
19 If ye be willing and obedient, ye shall eat the good of the land:
20 But if ye refuse and rebel, ye shall be devoured with the sword: for the mouth of the LORD hath spoken it.

21 How is the faithful city become an harlot! it was full of judgment; righteousness lodged in it; but now murderers.
22 Thy silver is become dross, thy wine mixed with water:
23 Thy princes are rebellious, and companions of thieves: every one loveth gifts, and followeth after rewards: they judge not the fatherless, neither doth the cause of the widow come unto them.
24 Therefore saith the Lord, the LORD of hosts, the mighty One of Israel, Ah, I will ease me of mine adversaries, and avenge me of mine enemies:
25 And I will turn my hand upon thee, and purely purge away thy dross, and take away all thy tin:
26 And I will restore thy judges as at the first, and thy counsellors as at the beginning: afterward thou shalt be called, The city of righteousness, the faithful city.
27 Zion shall be redeemed with judgment, and her converts with righteousness.
28 And the destruction of the transgressors and of the sinners shall be together, and they that forsake the LORD shall be consumed.
29 For they shall be ashamed of the oaks which ye have desired, and ye shall be confounded for the gardens that ye have chosen.
30 For ye shall be as an oak whose leaf fadeth, and as a garden that hath no water.
31 And the strong shall be as tow, and the maker of it as a spark, and they shall both burn together, and none shall quench them.

"If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness." 1John 1:9

Both the Old and New Testaments agree; we must confess and forsake our sins. If you are dirty and want to be clean, it is because you have actual dirt on you.  You can not become clean and still keep the dirt on you.  If you wash the dirt it will not change, it is still dirt and you are still dirty.  The only way to become clean is by washing the dirt away.  If the dirt comes back again, you are dirty again and you must start the cleaning process all over again.  Victory over sin is possible through Jesus, He will forgive us (justification) and He will clean us by taking away the sin (sanctification), but we have to give our sin to Him.  Our sin becomes His and we have no right to it as long as we wish to be cleansed from all unrighteousness.