We may speak with authority if what we say is true according to God's Word and when our character is Christlike.
And when we are relying on God's strength through our weakness. God has taught me a valuable lesson over the past 2 weeks; and that is we can reach a point in our spiritual maturity where we start feeling a bit cocky with the power and authority given to us through Jesus' sacrifice and resurrection. We need to take care to not think we are untouchable by the satanic influence because we've got the "power". Sometimes God has to allow us to realize just how weak we are before we can rise up again. As was explained to and through Paul:
"But He said to me, “My grace is sufficient for you, for power is perfected in weakness.” Therefore, I will most gladly boast all the more about my weaknesses, so that Christ’s power may reside in me. So I take pleasure in weaknesses, insults, catastrophes, persecutions, and in pressures, because of Christ. For when I am weak, then I am strong." ~ 2 Corinthians 12:9-10