What does 2 Corinthians 6:18 mean?

Paul is quoting from a collection of verses in the Old Testament Scriptures to make the connection between God's relationship with and expectations for His people Israel under the Old Covenant and all who are His through Christ under the new covenant.

God had redeemed Israel from slavery and commanded the Israelites to live separate from other nations. That meant keeping themselves pure from sin, including the worship of other nations' gods. God has redeemed those who are in Christ from slavery to the law and to their own sinfulness. Now Paul is urging believers to separate themselves from any connections to the worship of idols or ongoing participation in sin.

Paul references Isaiah 43:6 to finally establish that a believer's relationship with God through faith in Christ is that of a child to a Father. God calls Christians His sons and daughters. His commands to separate from the worship of idols and ongoing sin is not about religious rule following for its own sake. It is about being in a loyal and loving relationship with our Father, who is God.