We cannot actively live the Christian life and do the will of God successfully by human strength alone. While the Spirit empowers us for service, which is vital, His first work in us is to enable us to live the Christian life itself.
The Christian walk is not a natural life; it must be lived supernaturally, as Galatians 5: 16 explains: "This I say then, walk in the Spirit, and ye shall not fulfil the lust of the flesh."
If the disciples had relied solely on the knowledge and experiences gained from following Jesus for three and a half years, without the enabling of after Jesus' death, their knowledge would have amounted to nothing.
Initially, they were fearful and uncertain. But when the Holy Spirit came upon them, they were emboldened to proclaim God's word without fear and also carry out His perfect will.