The Law's promises are made on condition, whereas the Gospel promises are made without the condition

"Law and Gospel differ, in the third place, by reason of their promises. What the Law promises is just as great a boon as what the Gospel promises, namely, everlasting life and salvation. But at this point we are confronted with a mighty difference: All promises of the Law are made on certain conditions, namely, on the condition that we fulfill the Law perfectly. Accordingly, the promises of the Law are the more disheartening, the greater they are. The Law offers us food, but does not hand it down to use where we can reach it. It offers us salvation in about the same manner as refreshments were offered to Tantalus in the hell of the pagan Greeks. It says to us indeed: 'I will quench the thirst of your soul and appease your hunger.' But it is not able to accomplish this because it always adds: 'All this you shall have if you do what I command.'
Over and against this note the lovely, sweet, and comforting language of the Gospel. It promises us the grace of God and salvation without any condition whatsoever. It is a promise of free grace. It asks nothing of us but this, 'Take what I give, and you have it.' That is not a condition, but a kind invitation." - C.F.W. Walther, The Proper Distinction Between Law and Gospel, Pg. 10