I think it is amazing that so much was accomplished by the same disciples who denied Christ, doubted His resurrection, and huddled together in fear. This struggling and motley crew were the ones who built upon Jesus' "way" that led right down history's path to today's church. Their past faith failures did not define how they became the living manifestation of God's Love to others.
Every day, including today, is a day that you can step out as a bold disciple, even if yesterday or last week your faith was lukewarm.
When you trust the abiding presence of Christ you may find yourself activated (like Peter) with words that are confident and authoritative. You won't need to knit your brow trying to discover the most compelling testimony -- Just as Peter spoke from his own experience, you can speak from yours.
Peter's impact upon others grew exponentially AFTER he hit the bottom of those moments when he denied Christ (three times!) For many of us, our greatest potential for service to others flows directly from some aspect of inadequacy, failure, or suffering.
Consider your faith walk. Can you identify some time or some pattern that was unfaithful, doubtful or riddled with anxiety? If that time was in the past, what were the marks of faith's return? How did "belief" activate your words or actions? If you are currently experiencing a faith lapse, pray for renewed openness to Christ's presence in whatever situation you encounter.