In Holy Book of I Corinthians 7:12, St. Paul gives a command saying, "But to the rest I, not the Lord, say..." How could he say "I, not the Lord"? Does this mean a man who has a non-believing wife can divorce her since St. Paul said so and not the Lord?
In verse 12-15 (contrast 10) St. Paul says, on a particular case, "I, not the Lord", whereas on the main point, he said "not I but the Lord". The Lord has given commands concerning divorce (Mt 5:31); now St. Paul speaks with the authority given to him saying "I, not the Lord", declaring that he has no express revelation, or tradition delivered from the Lord, commanding it, but he gives his judgment; viz., under the ordinary inspiration which accompanied the apostles in all their canonical writings (1 Cor 7:40; 14:34; 1 Thess 4:15).
St. Paul, therefore, is not drawing a line between the authoritative commands of Christ and his own. Rather, he himself is claiming inspiration and the authority to set forth doctrine and practice (1 Cor 7:12-25). He has the "Spirit of God" (1 Cor 7:40).
To read more about this topic, please check: http://www.suscopts.org/literature/conference/2002lecture2.pdf.