What does it mean to believe?
the "first use" of a word in Scripture is often very significant as it establishes the primary or most significant meaning, and this general principle is true in the Spirit's inspired use of 'aman in Genesis 15:6 where "Abram believed in Jehovah" = Hebrew verb 'aman = Greek verb
Genesis 15:6 Then he believed (Aman in the Hiphil stem) in Jehovah; and He reckoned it to him as righteousness.
Comment: The use of aman in this passage indicates that Abram did not just give mental assent to God's promise ( Ge 15:5 ), but that he relied on that promise and made a personal commitment. In other words Abram's faith was not only cognitive (the mental act of acquiring knowledge) but also personal for he believed God’s word of promise (Cp the NT "commentary" on this OT use of "aman" -- Hebrews 11:8 = His faith was the root of the fruit of obedience, which is a sign that one's faith is genuine saving faith and not just mental assent! the depth of his faith which led to the ultimate act of obedience in Heb 11:17 , 18 , 19 ) and he committed his soul to the God Who had revealed Himself. Considering that "amen" (truly, it is certain) is derived from "aman", it is as if Abram heard God's promise in Ge 15:5 and said "Amen!" Jesus repeatedly used the Hebrew word "amen" ("Truly") to express the trustworthiness and abiding certainty of His sayings.