"This passage, termed the Longer Ending to the Marcan gospel by comparison with a much briefer conclusion found in some less important manuscripts, has traditionally been accepted as a canonical part of the gospel and was defined as such by the Council of Trent." [source]
I didn't know that, or if I did at one time, I've since forgotten it.
Interesting tidbit: Every reference I've checked so far indicates Luther treated the long ending as Sacred Scripture.
A contemporary Luther's, the great Roman Catholic theologian Cardinal Cajetan,doubted the authenticity of Mk 16:9-20.