Mark 16:15-16 is in reference to the gospel of
Acts 2 preached on Pentecost, when the Holy Ghost was poured out (with tongues) as the earnest of our inheritance.
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The
Mark 16:17-18 signs that follow believers are the gifts of the Spirit spoken of in
1 Corinthians 12-14.
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Believers will obey the gospel and receive the The gift of the Holy Ghost. (tongues as initial evidence)
After which,
The gifts of the Spirit will manifest in various members of the church body
1 Corinthians 12, as Spirit chooses to operate.