Charles Spurgeon Commentary 1 Samuel 20:3

"And David sware moreover, and said, Thy father knoweth well that I have found favor in thine eyes; and he saith, Let not Jonathan know this, lest he be grieved: but truly as Jehovah liveth, and as thy soul liveth, there is but a step between me and death." — 1 Samuel 20:3 (ASV)

And David sware moreover, and said, Thy father certainly knoweth that I have found grace in thine eyes; and he saith, Let not Jonathan know this, lest he be grieved: but truly as the LORD liveth, and as thy soul liveth, there is but a step between me and death.

He wanted Jonathan to believe the truth, namely, that Saul was seeking to kill him, and that he was in great danger from the wrath of the king; therefore he took a double oath that it was indeed so. It is not for Christians to imitate David in this respect, for our Lord's command to his disciples is, Swear not at all; but let your communication be, Yea, yea; Nay, nay: for whatsoever is more than these cometh of evil.