Baptism is not the door to our heavenly mansion but the gate to the path that leads to the mansion. Relatively few are genuinely interested in entering the gate and walking that path (1 Nephi 14:12; 2 Nephi 31:17–20; 3 Nephi 27:33; D&C 132:25).
The words strait and narrow mean about the same thing: constricted, tight. The juxtaposition of synonyms is a familiar ancient literary technique.54
A modern use of the word strait is the Strait of Magellan (in Patagonia, at the southern tip of South America), which connects the Pacific and Atlantic Oceans. It is not a straight line between two points—one cannot see one ocean from the other. So with us: we cannot see the other side (of heaven), but if we persevere in the strait path, we will arrive there.
Note the scriptural difference between “strait” and “straight”; they are two different words. Our path is described as “strait”; the Lord’s path is “straight” (Alma 7:9; 37:12; D&C 3:2). Jesus Christ is the only one who never had to make course corrections. Our Savior’s paths are always straight because “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” (Alma 7:20).