Became futile (3154) (mataioo from mataios = empty, vain, mataios being used in the Septuagint (LXX) for the Hebrew word habal (01891) rendered vanity or vanities, and frequently used to describe to idols - see Vine's note below) means to become futile in one's thinking, to be given over to worthlessness, to think about worthless things or to become destitute of real wisdom. Paul's point is that their ideas and conceptions of God had no intrinsic value corresponding with the truth. This is the only NT use of mataioo but there are 6 uses in the Septuagint (LXX) (1Sa 13:13; 26:21; 2Ki. 17:15; 1Chr. 21:8; Jer. 2:5; 23:16). 2 Kings 17:15 And they rejected His statutes and His covenant which He made with their fathers, and His warnings with which He warned them. And they followed vanity (idols) and became vain (Lxx = Mataioo), and went after the nations which surrounded them, concerning which the LORD had commanded them not to do like them. Jeremiah 2:5 Thus says the LORD, "What injustice did your fathers find in Me, That they went far from Me And walked after emptiness (Lxx = mataios) and became empty (Lxx = mataioo)? Godet writes... The understanding was reduced to work in vacuo. It rendered itself in a way futile (Romans Commentary online)