Had Abraham just said “oh I’ll give you Isaac” but never climbed the mountain, built the altar or lifted his hand to sacrifice him, there would be room for us to doubt his genuineness.
But, Abraham walked up the mountain with his son, fully intending to keep God first in his life. He was so set on honoring God above all others that when the time came for him to live out those intentions he didn’t hesitate. Because of this display of obedience there isn’t room for us to doubt Abraham’s devotion to God.
Abiding by your words takes your witness to an entirely different level. Make certain that you are living what you are preaching.