Adam Clarke on Mark 16:17

These signs shall follow - Or rather, accompany; this is the proper import of the original word παρακολουθησει, from παρα with, and ακολουθεω, I follow.

Them that believe - The believers, as we express it; i.e. the apostles, and all those who in those primitive times were endued with miraculous powers, for the confirmation of the doctrines they preached.

In my name - That is, by the authority and influence of the almighty Jesus.

Cast out devils - Whose kingdom Jesus Christ was manifested to destroy.

Speak with new tongues - This was most literally fulfilled on the day of pentecost, Acts 2:4-19.