David sware — The matter being of great moment, and Jonathan doubting the truth of it, he confirms his word with an oath, which follows in the end of the verse. Only he interposeth a reason why Saul concealed it from Jonathan.
Related Commentaries of 1 Samuel 20:3
1 Samuel 20:3
3 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.