It is not enough to have grace, but this grace must be kept in exercise. The Christian's armour is made to be worn; no laying down, or putting off our armour, till we have done our warfare, and finished our course. Our armour and our garment of flesh go off together; then, indeed, will be no need of watch and ward, shieled or helmet. Those military duties and field-graces--as I may call faith, hope, and the rest--shall be honourably discharged. In heaven we shall appear, not in armour, but in robes of glory. But here these are to be worn night and day; we must walk, work, and sleep in them, or else we are not true soldiers of Christ. This Paul professeth to endeavour. 'Herein do I exercise myself, to have always a conscience voice of offence toward God and toward men,' (Ac 24:16). Here we have this holy man at his arms, training and exercising himself in his postures, like some soldier by himself handling his pike, and inuring himself before the battle. (Gurnall, William, and J. C. Ryle. The Christian in Complete Armour. Banner of Truth Trust, 2002. pages 63-64)
Paul told believers to put on the whole armour of God because it is likely we have failed to appropriate the power of the Holy Spirit given to us to protect our minds and hearts. Many times we have walked around without traction and support of the Gospel of peace which is like combat boots or God's truth which is like a belt. We have wandered unwittingly into a conflict without the shield of faith or left the sword of the Spirit (which is the word of God) in the sheath or at home. We are able to "be" strong because this rests completely on Who Jesus is and the new creations He has made us to be. Our identity in Christ does not mean we always or are currently using the full range of protection afforded us by God's grace, and thus we are called to put on the whole armour.
While soldiers sharpen their swords, knives and bayonets for hand-to-hand combat, we are told the manner which is to mark our preparing for and engaging in battle in Ephesians 6:18: "...praying always with all prayer and supplication in the Spirit, being watchful to this end with all perseverance and supplication for all the saints..." If we are not praying, it is likely victory will be an ideal instead of the continuous reality in practice. We are told to pray without ceasing and in everything give thanks because this is the will of God in Christ Jesus concerning us. Jesus told His disciples in Matthew 26:41, "Watch and pray, lest you enter into temptation. The spirit indeed is willing, but the flesh is weak." When it comes to the spiritual conflicts, the Holy Spirit is able and willing to protect us from all harmful defeats. If we are unwilling to pray, refuse to look to the LORD Jesus or yield to the Holy Spirit in obedience, we cannot stand in the power of God's might.