Alma is making a scriptural reference in this verse. He introduces the concept of the paths as a logical outgrowth of the dilemma of Zarahemla. A dilemma has at least two aspects, and Alma visualizes them as paths that must be walked. He uses this path imagery to further compliment his audience by making a reference to Isaiah:
3 The voice of him that crieth in the wilderness, Prepare ye the way of the LORD, make straight in the desert a highway for our God.
4 Every valley shall be exalted, and every mountain and hill shall be made low: and the crooked shall be made straight, and the rough places plain:
5 And the glory of the LORD shall be revealed, and all flesh shall see it together: for the mouth of the LORD hath spoken it.
When Alma tells his audience that “I perceive that ye are making his paths straight,” he is very directly telling them that the choices they are making are preparing them for the coming of the Atoning Messiah.
20 I perceive that it has been made known unto you, by the testimony of his word, that he cannot walk in crooked paths; neither doth he vary from that which he hath said; neither hath he a shadow of turning from the right to the left, or from that which is right to that which is wrong; therefore, his course is one eternal round.
Rhetorical: Alma takes the basic image of “straight paths” and now reapplies the image. Where the original intent was a metaphor for the preparation for the Atoning Messiah (which context he did use) he now restructures the metaphor into a description of the path of the Lord itself. This is no longer the prediction or preparation for the Lord, but the experience of one who is walking in the path of the Lord. This path is already “straight” in that “he cannot walk in crooked paths; neither doth he vary from that which he had said…”