Ephesians 6:10-20 talks about the whole armor of God. To have a chink in your armor means to have a weak link or a damaging weakness. Like the Achilles heel it is a fatal weakness in spite of overall strength leading to a downfall. So even if we are strong because of the Lord in us if we give place to sin it will lead to our downfall.
We cannot be strong and not give place to the enemy without the Lords help. The armor is what God gives us to protect ourselves against the enemy of our souls. We need to make sure there are no chinks in our armor.
We do this by being strengthened in our inner man spoken about in Ephesians 3:16. We need to address sin in our lives immediately. If we don't, it will get worse. We are told in 2 Corinthians 13:5 to examine ourselves as to whether you are in the faith. Test yourselves. Do you not know yourselves that Jeus Christ is in you unless indeed you are disqualified.
It only takes a small chink left unattended to become a stumbling block in your relationship with God. God gives us His solution and that is to come to the cross. We need to come to the cross frequently to avoid this from happening. A life of holiness and sanctification is vital in preventing this. If just one thing is wrong in our Christian walk this is the place the enemy will strike.
If you have a chink in your armor God is passionately pursuing you so that you can receive His grace and unfailing mercy.