Certainty goes no where without Necessity. Even so, it is heresy to say that a true Christian continuing in holiness does not continue thus because God made it certain he would. Necessity goes no where without Certainty. Have not one truth of God to the exclusion of all other truths of God. The God has no power to promise an end who holds no power to manipulate the means in such a way as to accomplish His end.
Certainty proves necessity, and vice versa.
If a thing is certain to happen, then it must necessarily happen. If a thing necessarily happens, then it was certain to happen.