Mk 16:16 is just as genuine as any other verses in the bible.
1 Cor 15:3,4:
For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures;
And that he was buried, and that he rose again the third day according to the scriptures:
Christ died for every man, Heb 2:9, is Paul saying every man will be saved unconditionally by Christ's death, burial and resurrection? No. The gospel is something that must be obeyed, and only those that obey the gospel will be saved, Rom 10:16; 2 Thess 1:8; 1 Pet 4:17, those disbedient to the gospel will be lost.