God: "I looked for someone to take a stand for me, and stand in the gap" (Ezekiel 22:30)
Oath-taking is permitted, but it is not encouraged. In civil life, as in a courtroom, oath-taking is permitted. And when one is put under oath, he or she is not sinning against Christ’s teaching. Also, on rare occasions such a practice may be necessary, as it was for Paul. This said, oaths are not to be a part of everyday conversation. Christians should not need such devices. They should be known to be people of truth.
Salvation is achieved either through our work or God’s work. And since salvation is only and entirely a work of God, anything we relied upon for merit actually competes with and cancels out the work of God.