1 Corinthians 12:9 (KJV)
To another faith by the same Spirit; to another the gifts of healing by the same Spirit;
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This seem like a simple answer but this section is talking about extra ordinary gifts. It is an unusual gift of faith to some believers only, not general faith in Christ. What these gifts were exactly and if they are still given today is a controversy within Christianity. Primarily the split is between charismatics and tradition cessation movements.
For my own part I think this gift was and is a unusual surge of faith in a believer to believe the gospel in an unusual manner where the requirement is great and needy for a situation. For example, Martin Luther must have had this gift to stand almost alone fully relying on salvation apart from works while the established church called him an accursed heretic. This was an unusual faith that benefited the church at large. It was an extra ordinary gift given by the Spirit on Luther.
Many of the early believers in Acts seemed to have this gift, especially those faithful while being martyred. Similarity when David killed Goliath, or Samson slew the Philistines, it would seem they had unusual faith given to them. Although not possessing them against their wills like Devils, the Holy Spirit can surge through willing vessels, where faith is needed, in remarkable ways.