Here the 1841 British edition accidentally replaced this with the in the phrase “this great joy”. The phrase “the great joy” is odd here because for such a phrase headed by the we expect some kind of postmodification, as in Alma 7:4: “the exceeding great joy of knowing that they are established again in the way of his righteousness”.
The two following LDS editions (1849 and 1852) followed the reading with the in Alma 22:15, but in the second printing of the 1852 edition the original this was restored (in the stereotyped plates), probably by reference to the 1840 edition. The critical text will here maintain the earliest reading, “this great joy”.
Summary: Maintain in Alma 22:15 the demonstrative this in “this great joy”.