They are often used with the direct object “you,” as when Paul tells the Romans “For I am longing to see you so that I may share with you some spiritual gift to strengthen you” (Romans 1:11). But there are lots of other things that get strengthend in the NT: “[Paul and Barnabas] strengthened the spirits of the disciples and exhorted them to persevere in the faith” (Acts 14:22). “Judas and Silas, who were themselves prophets, exhorted and strengthened the brothers with many words (Acts 15:32)”. “[Paul] traveled through Syria and Cilicia bringing strength to the churches (Acts 15:41).”
So it seems that we, like our brothers and sisters in the early church should be about strengthening one another. The other word for confirm or strengthen, parakaleo (which gives us "paraclete"), is based on the idea of someone’s being called to another’s side as a helper. But sterizo has a very different feel to it. Like parakaleo sterizo also means “strengthen,” but it is based on the root ster-, which means “firmness, solidity, strength” -- and gives us the word "steroid".
I smiled at the image of Christians “bulking up” their sisters and brothers, making them firm for the fight. No side effects, either, except that the person administering this kind of steroid is also going to benefit from it as much as the one receiving it.
Sterizo can also be found in Lk 16:26 and 22:32; Acts 18:23; Rom 1:11 and 16:25, and in 1 Thess 3:2 and 3:13.