Contradiction with John 1:12
While 2 Corinthians 6:18 implies God's selection of specific people as His children, John 1:12 indicates that anyone who receives Christ can become a child of God.
John 1:12: But as many as received him, to them gave he power to become the sons of God, [even] to them that believe on his name: [power: or, the right, or, privilege]
Contradiction with Romans 8:14
2 Corinthians 6:18 conveys a promise or future state of being God's children, whereas Romans 8:14 states that those led by the Spirit of God are already God's children.
Romans 8:14: For as many as are led by the Spirit of God, they are the sons of God.
Contradiction with Galatians 3:26
2 Corinthians 6:18 highlights a promise made to believers, but Galatians 3:26 suggests believers are already children of God through faith in Christ Jesus.
Galatians 3:26: For ye are all the children of God by faith in Christ Jesus.
Contradiction with Matthew 5:9
While 2 Corinthians 6:18 explicitly states God's declaration of fatherhood, Matthew 5:9 offers another pathway to being called children of God through peacemaking.
Matthew 5:9: Blessed [are] the peacemakers: for they shall be called the children of God.
Paradox #1
One possible moral inconsistency with this verse is the conflict some people see between the inclusivity of all people being "children of God" and the exclusive nature of being "separate" from non-believers, which can seem divisive. Some might feel this contradicts the general message of love and acceptance found in other parts of the Bible.