Who says we are qualified to preach and by whose authority?
I don’t know if you are qualified to preach.
Many teach that “preaching” and “teaching” is reserved to those elders recognized by the local church. This applies to the regular flock-tending preaching of an elder/pastor.
The preaching of the cross to dead people is not the systematic exposition of Scripture and therefore dos not warrant the oversight that attends the office of elder. In Scripture, people went joyfully to tell others about Christ; often in response to being healed physically. How much more should those who are healed spiritually go joyfully telling others about Christ?
THIS preaching - the proclamation of the saving Gospel of Christ - is the duty and privilege of every Christian. This is what I think John means in 1 John 4:13-15 “Hereby know we that we dwell in him, and he in us, because he hath given us of his Spirit. And we have seen and do testify that the Father sent the Son to be the Saviour of the world. Whosoever shall confess that Jesus is the Son of God, God dwelleth in him, and he in God.”