Mastering strength as a virtue in the spiritual life can be tricky. I say this because when most of us think of strength, what comes to mind are characteristics of one's ability to firmly assert oneself in an entirely self-sufficient manner. Spiritual strength as a virtue, which helps us to persevere in charity and remain firmly rooted in peace of mind amidst assaults from within and without, is fostered in an entirely different manner than that image of strength with which we are familiar.
There are many people whom others would describe as strong-willed. We may ask ourselves if it is these who are more well disposed to strength from God as their nature need only be purified and perfected by grace. I'm sure that, fortunately, this is the case with many. On the other hand, one's natural strong-mindedness may also work against the soul's growth as it often espouses pride and self-reliance. Conversely, those whom we would describe as impressionable and weak when faced with adverse moral or spiritual pressures may, with self-knowledge, be at an advantage in relying totally on God for strength. They also have the assurance, "God chose what is weak in the world to shame the strong" (1 Cor 1:27). However, the naturally weak-willed more acutely face the danger as one "sown on rocky ground...who hears the word and immediately receives it with joy; yet he has no root in himself, but endures for a while, and when tribulation or persecution arises on account of the word, immediately he falls away" (Mt 13:20, 21). I would say what makes one more well-disposed to strength from God is the firmness of one's faith and trust in God, as opposed to whether one is naturally a strong-willed person, as I describe further below.
The well-known phrase of St. Paul's, "For when I am weak, then I am strong" (2 Cor 12:10), immediately informs us against the popular notion of strength as being a matter of self-sufficiency. Strength as virtue relies entirely on the Lord's providing it for us. Paul intimates that it is during times of utter hardship, when he has little choice but abandon himself to the Lord, when the Lord's "power is made perfect in weakness" (2 Cor 12:9).
There seems to be a contradiction in the notion that in remaining weak we are made to be strong. It seems that any strength we receive from the Lord would not in any way manifest itself in ways we would attribute to the virtue, for then there would be little room for weakness. Yet the strength the Lord provides us on our journey through this life is to be constantly used to ensure steadfastness and perseverance in doing good and resisting or enduring evil. With his endurance of "weaknesses, insults, hardships, persecutions, and calamities" (2 Cor 12:10), Paul of all people, knew this. This contradiction seems apparent because we would still be attributing to strength characeteristics associated with human strength, as opposed to that which comes from the Lord.
Scripture draws a distinction between human and divine strength. David contrasts those who boast in horses and chariots as opposed to boasting in the Lord for help in battle (see Ps 20:7). David also sings,
"A king is not saved by his great army; a warrior is not delivered by his great strength ... The eye of the Lord is on those who fear him, on those who hope in his steadfast love, that he might deliver their soul from death, and keep them alive in famine." (Ps 33:16).
I would note here, too, that the distinction between human and divine strength is not merely a matter of distinguishing physical from spiritual strength. The psalms are absolutely riddled with prayers to God for strength. God himself is addressed with the the very title, "O my Strength" twice in Psalm 59. Much of the time, too, the strength invoked is that needed for fighting or protection in physical battle (see, for example, Ps 18:39; 55:18; 68:25, 38; 118; 140:7; 144:1). (Though these same psalms may very well be prayed as petitions for spiritual strength.) The difference is that this strength - physical or spiritual - came from God, as his strength, and by trusting in him for it rather than by trusting in oneself or in others: "it is better to take refuge in the Lord than to put confidence in man" (Ps 118:8).
In the book of Isaiah, God tells Israel that, as opposed to attaching itself as a nation to other, stronger nations for strength and security, that the people of Israel will only find its strength in turning back to God himself. "In returning and rest you shall be saved, in quietness and trust shall be your strength" (Is 30:15).
"In quietness and trust shall be your strength." Strength from God goes hand in hand with a degree of quietness, or serenity, of soul. The consistent quietness of the soul in which God's peace is firmly established finds little disturbance in fear of trial or in face of situations demanding heroic courage. "Fear not, for I am with you" (Is 41:10). Persons face and work through such demanding situations using the strength which God provides them. It is this very strength, too, which safeguards and maintains the quietness of the soul in which the strength itself blossoms. This quietness as its conveyed in Isaiah may be also related to the silencing of ambitious thoughts of self-reliance. Such tranquility resides in the heart through humble submission to God's dominion over one's life. This dominion is that for which we pray when we say, "thy kingdom come"; and it is what allows God to take over and transform the feeble heart, infusing it with his own strength.
Besides quietness, the book of Isaiah relates that Israel's strength will also result from trust. Strength as a virtue comes from God, as opposed to oneself, and so, if strength is to remain constant in one's soul, it will result from trust in him for that strength. Strength may very well be the immovable trust itself which confides that all creation continues to exist in and through Christ (Rom 8:6, Col 1:17) and that God himself will direct life's affairs according to his will, using us as his willing instruments when he sees it fit. Such trust in God stems from steadfast faith that God is always with us, that he himself is our strength (Ps 73:26). Such faith enables God to act in us and carry us through the most demanding of situations. "I can do all things through him who strengthens me" (Phil 4:13).
Knowledge of God's faithfulness enlivens such faith. God wants to provide us with the strength we need to faces life's demands; we know this because he has promised such faithfulness (Ps 89:24, 33; Is 61:8; Jer 31:3; 1 Thes 5:23, Heb 10:23). Those who strive to remain in God's grace may be confident of his readiness to help them (Deut 7:9, Ps 25:10). And, if we have fallen out of God's grace, we remain confident that his steadfast love for us will continue to warrant such faithfulness when we turn back to him (2 Chr 7:14, 2 Tim 2:13, 1 Jn 1:9). We direct our trust to such initiative on God's part, that he himself will be our strength.
The Lord is my strength and my shield; in him my heart trusts; so I am helped, and my heart exults, and with my song I give thanks to him. (Psalms 28:7)