He began when the ox was a tiny calf to carry it a mile every day, and the increase of weight was so gradual that he did not feel it; his arms became used to the weight, and as the ox grew bigger, he at the same time grew stronger.
Strength of body then grows and increases in proportion to our use of it.
So, too, does strength of mind.

Here is a boy, born with good abilities and with an intelligent mind.

Take that child, and shut him off from every possibility of using his mind; never teach him anything, never allow him to look at a book or a picture, keep him shut off from everything that might tend to open his mind, tell him nothing, bring him up as a mere animal, and soon he will lose all his powers of mind, and become an imbecile.

But, on the other hand, teach him, train him, educate him, let his mind have full scope and exercise, and his mental powers will grow and increase a hundred-fold, for strength of mind, like strength of body, grows with the using.
Just so is it with strength of soul.

Every temptation you overcome makes you stronger, every lust you subdue, every battle of soul you fight, every inclination to evil you resist, makes you stronger.
'From strength to strength' is the motto of the Christian.
So let us press forward.
'Till we all come in the unity of the faith, and of the knowledge of the Son of God, unto _a perfect man_' (or as R.V.has it, a _full-grown man_) 'unto the measure of the stature of the fulness of Christ.' Now we are but children in spiritual strength, then we shall be giants in power, full-grown men, with full powers and energy and strength, ready to work for the Master through eternity..