'Be Strong' in the Bible

Or what king, going to war with another king, will not first take thought if he will be strong enough, with ten thousand men, to keep off him who comes against him with twenty thousand?

But keep watch at all times with prayer, that you may be strong enough to come through all these things and take your place before the Son of man.

But I prayed earnestly for you, that your faith would not fail [to be strong]. And when you have turned back [i.e., from going astray briefly], provide [spiritual] support to your brothers."