From the outside, Jerusalem’s walls seemed invincible. All the kings and people of the earth agreed that there was no way to get in from the outside (v.12). The Bible Knowledge Commentary says this about Jerusalem:
Jerusalem was a mighty fortress that had seemed secure. The city had been entered by invading armies on a few occasions previously (cf. 1 Kings 14:25–28; 2 Kings 14:13–14; 2 Chron. 21:16–17). But its defenses had been rebuilt and strengthened (cf. 2 Chron. 32:2–5; 33:14), and a water supply into the city was established with the digging of Hezekiah’s tunnel. So by Jeremiah’s time the kings considered the city impregnable.
The thing that brought Jerusalem down was not what was happening on the outside, but what was happening on the inside. Lamentations 4:13 says it was “the sins of her prophets and the iniquities of her priests.” It was also due to trusting other nations (i.e. Egypt) to help them rather than God. Their king (Zedekiah), the “anointed of the Lord” was also to blame.
The same is true for us as well. Any external attack on a child of God only succeeds if there is a problem with the internal defenses. We have an armor that can quench all the fiery darts of the wicked, but if we fail to put it on, we will not stand in the evil day.
What is going on inside your heart? How are you exposing yourself to the attacks of Satan by not properly guarding your heart? Every failure beings not with an external attack but with internal sin, laziness, or longing. James 1:14 puts it this way…
James 1:13-16 “Let no man say when he is tempted, I am tempted of God: for God cannot be tempted with evil, neither tempteth he any man: But every man is tempted, when he is drawn away of his own lust, and enticed. Then when lust hath conceived, it bringeth forth sin: and sin, when it is finished, bringeth forth death. Do not err, my beloved brethren.”