Is Paul usurping the Lord's authority in I Corinthians 7:12?

Paul is discussing marriage in this chapter. The reason I wanted to discuss the matter is because I'm sure there is confusion regarding the fact that Paul says in verse 12 that, "To the rest I say (I, not the Lord) that if any brother has a wife who is an unbeliever, and she consents to live with him, he should not divorce her. If any woman has a husband who is an unbeliever, and he consents to live with her, she should not divorce him.

For the unbelieving husband is made holy because of his wife, and the unbelieving wife is made holy because of her husband. Otherwise your children would be unclean, but as it is, they are holy. But if the unbelieving partner separates, let it be so. In such cases the brother or sister is not enslaved. God has called us to peace. For how do you know, wife whether you will save your husband? Or how do you know husband if you will save your wife?

When Paul states in verse 12 to the rest I say and not the Lord, he is by no means giving his personal opinion on the matter. Rather he is providing new revelation never dispensed before regarding marriage. If you read verses 10-11 in this chapter Paul talks about if one is married, do not separate from your spouse. For if you do, remain unmarried or reconcile with your spouse. Understand this particular command came from the Lord Jesus. Jesus stated the following, "It was said,'Whoever divorces his wife, let him give her a certificate of divorce.' But I say to you (i.e. not Moses and the law, but I, Jesus say) that everyone who divorces his wife, except on the ground of sexual immorality, makes her commit adultery, and whoever marries a divorced woman commits adultery" (Matt 5:31,32). The part where Jesus declares that those that divorce and marry another commit adultery was brand new revelation at that time. Moses never made that declaration in the law. 

So do you see why it was important for Paul to differeniate between what Jesus commanded and the new command given to him from the Spirit of God to deliver to the church at Corinth?  What Paul wrote Jesus never spoke about. Let discuss the new revelation Paul gives regarding marriage.

1. Paul discusses that if an unbelieving spouse desires to remain with their believing spouse, let them remain. Do not get a divorce (vv 12-13).

2. But, if the unbelieving spouse desires to leave, for the sake of peace let them leave. The believer can re-marry in the Lord (vv 15, 39).

3. The believing spouse because the Lord's favor rests upon them, sanctifies [(not saves) their unsaved partner and children, vs 14)]. The unsaved partner can receive blessings they would not otherwise enjoy if they were married to an unbeliever like themselves. The saved spouse provides a holy covering for the unsaved spouse. Hopefully the unsaved acknowledges the blessings they are receiving because of their saved partner and becomes a believer in Christ by repenting and turning to the Lord by faith.

In conclusion, just because Paul said "I said" and not the Lord does not mean Paul is usurping the Lord's authority. Once again these particular truths Paul revealed were never spoken of by Christ. What this also proves is that the same Holy Spirit that led our Lord to declare absolute truth is the same Spirit that led Paul and leads all true believers. Paul's words had just as much force and power as Jesus' words for he was an Apostle inspired by the Holy Spirit.