The definition of strength, in this respect, is that it is the ability of the individual to express and have faith in the most valued qualities of its nature. If the individual can lose sight of such qualities, or knowingly resign responsibility for them, then this would be a mark of insufficient strength in that individual.
This type of strength is not the lower order one which we are more often aware of in this world; which is considered to be, like a steam roller, concerned to go its own way at the expense of crushing and pushing aside all opposition. This is not strength but force and, while the former is allied to spiritual integrity, the latter is only concerned with having its own way...
Another way of drawing attention to the significance of strength is to realise that, if we believe that we possess a particular valuable quality, but discover that a situation arises in which we lose it, then that quality was never properly possessed by us. In other words, we only properly possess qualities of character when they are so embedded in our nature that they can never be lost. Such ‘possessions’ are an intrinsic part of our ultimate unique Divine Individuality and we are not strong in that reality until it is unassailable.
It would be quite wrong for us now to jump to the conclusion that God has encouraged ‘evil’ in His University in order to supply this very encounter of opposition as a part of the curriculum. The reality must be much more subtle than this, and very important for us to understand.
It would be a harsh and ungracious person who would create a system of education in which he deliberately cultivated evil for his children to face. It would not only be a violation of love towards the children who opposed it, but it would be an even greater violation of love towards those in whom the evil was cultivated.
But there is another way in which evil can arise in the educational system which does not contradict the nature of the educator, and that is to allow the full function of freedom of initiative, invention and exploration, for this will surely lead to mistakes occurring, out of which the quality of evil will quickly grow, as we have mentioned before. In this way, the recognition of the wrong use of our Divine Nature, and the experience of the unpleasantness of the repercussions, will lead to a desire to correct the error.
Thus the full cycle of divine integrity and responsibility can be experienced in which error breeds evil, which is recognised as distasteful and which leads to the discovery of the means of correction.
...So we have arrived at the need for, and proper allowance for, the principle of trial and error.
I have William Arkle to thank for the clearest understanding of what God was aiming for when He designed (created) this mortal life; and (from the opposite, human, perspective) how this actual life as we personally experience it, can be understood as well-designed for God's purposes.
Here Arkle is talking about the need for 'strength' which can also be termed 'courage' - and the distinction between mere personality-level strength/ courage (in service of within-mortal-life goals); and that divine and spiritual strength/ courage which has to do with preparing us for heavenly life as children of God.
It is important to recognize that: we only properly possess qualities of character when they are so embedded in our nature that they can never be lost.
But Arkle understood Men as having to develop and 'perfect' this indomitable strength through many situations in multiple incarnations. While I, on the other hand, believe that (because of the work of Jesus Christ) we only need to understand and desire this strength - and we do not personally need to achieve it in life.
When it comes to the possibility and choice of resurrection, after this mortal life; it is enough for us to recognise the need for such a fullness of spiritual strength - and it will be granted us by resurrection.
The purpose of learning in this mortal life is therefore to know what is Good: to learn True Values. It is neither an intention nor a possibility of this life - nor of multiple such lives - to attain any kind of perfection; whether in strength/ courage nor in any of the of the essential virtues.
Strength/ courage is offered, indeed, as an inseparable part of 'the total package' of resurrection; and resurrection is wanted by those who love Jesus Christ and wish to follow him to Life Everlasting (by means of our resurrection).
Strength is one necessary attribute of being divine - and the need for strength is something that some of us need to learn in mortal life; while others of us already know it before we are incarnated as mortals; or else our love of God and of divine creation is already so strong that it encompasses all the essential specific attributes.