Peter, John, and Acts 4:13

Acts 4:13
Now as they observed the confidence of Peter and John and understood that they were uneducated and untrained men, they were amazed, and began to recognize them as having been with Jesus.

For an individual to be used by God doesn't require them to be impressive in human terms. It's not a matter of how to fulfill a calling, it's a matter of who called you. If its God calling you, then you are of no lack, because adequacy is from Him.
 
2 Corinthians 3:4-6
Such confidence we have through Christ toward God. Not that we are adequate in ourselves to consider anything as coming from ourselves, but our adequacy is from God, who also made us adequate as servants of a new covenant, not of the letter but of the Spirit; for the letter kills, but the Spirit gives life.

Peter and John claimed their confidence through Christ, not by anything esteemed in human terms. And standing before the rulers and elders of the land... it was enough.

Please understand, this does not mean we shouldn't work towards excellence in everything we do for the sake of Christ. It is a means by which doubt is cast away, and true confidence is gained.

Hebrews 13:5-6
He Himself has said, "I WILL NEVER DESERT YOU, NOR WILL I EVER FORSAKE YOU," (Deut. 31:6) so that we confidently say, "THE LORD IS MY HELPER, I WILL NOT BE AFRAID. WHAT WILL MAN DO TO ME?" (Psalm 118:6)