He asked "can you imagine yourselves …" in three different situations:
"can you imagine to yourselves that ye hear the voice of the Lord, saying unto you, in that day: Come unto me ye blessed" (v. 16)
"Or do ye imagine to yourselves that ye can lie unto the Lord in that day, and say—Lord, our works have been righteous works upon the face of the earth" (v. 17)
"Or otherwise, can ye imagine yourselves brought before the tribunal of God with your souls filled with guilt and remorse, having a remembrance of all your guilt" (v. 18)
He paints a picture of the sinner at the last day, before the tribunal of God, and beholding their own countenances. What makes these words so impressive is that there is a visual dimension to them all. He is saying "Can you visualize this?" It is very concrete. We speak with the spirit when we are not so abstract but are very specific.