The following is an excerpt from A.T. Pierson's book, "Shall We Continue In Sin?":

And now, in one word, Paul turns our thoughts to the practical aspect of this union with Christ. What does all this mean, and how is this truth to be transformed into life? How is the believer to put this theory into practice? (Psalm 1; John 15). 

The answer begins now to be given and is found in one word: Reckon—the equivalent of another word, Count, which first appears in Genesis 15:6: “Abram believed the Lord, and He credited it to him as righteousness.” What Abram did that was counted as righteousness is clear from the original meaning—Abram amened God. When God declared something, though humanly impossible, Abram said, “Amen, it shall be so, just as God has said.” This act of faith, this saying Amen to God, is further described in Romans 4:3, 17-22. Refer to the entire passage. Compare this with Hebrews 11:8-19. 

In these passages, several phrases clarify the meaning of the word Reckon: “Against all hope, he believed in hope,” “he did not consider his own body, already dead since he was about a hundred years old, nor the deadness of Sarah’s womb.” “He did not waver at God’s promise through unbelief but was strong in faith, giving glory to God, being fully convinced that what He had promised, He was able to perform.”