It is a great comfort to know that God has his reserves. He is never defeated. He brings out his reserves when necessary. These may be physical things, as it says in Job 38:22/23:
"Hast thou entered into the storehouses of the snow, and hast thou seen the treasures of the hail,
Which I have reserved for the time of distress, for the day of battle and war ?"
If we go to Genesis we find that God speaks to Adam of the ground yielding thorns and thistles (Genesis 3:18). The ground would not have yielded such things in the garden of Eden, but God no doubt had such things in reserve somewhere unless we say he created them after Adam's fall.
Then we know that God has his angels. They are not normally seen by Man, no doubt because their dwelling is in the heavens (Mark 13:32; Luke 2:15), but they are there and available to be called on for service (Hebrews 1:13/14) .
However, what is just as important is that God has persons in reserve. In 1 Kings 19:14-18 we find Elijah saying effectively that he was the only adherent of Jehovah left, when in fact there were seven thousand that had not bowed the knee to Baal. They were not evident to Elijah but Jehovah knew them, in a way analogous to what we get in 2 Timothy 2: 19 "[The] Lord knows those that are his." Previously we find that Elijah had been fed by ravens and then by a widow woman who was not of Israel (1 Kings 17; Luke 4:25/26)
In Jeremiah we find that when the prophet was put in the dungeon there was one who was prepared to speak up for him: Ebedmelech the Ethiopian (Jeremiah 38:7-13). Subsequently Ebedmelech had a special word from Jehovah for himself (Jeremiah 39:15-18). This is all the more remarkable because Ebedmelech was a foreigner and probably a black man.
When we come to the New Testament we find there were godly persons living around the time of the birth of Christ. Thus we have: Joseph and Mary; Zacharias and Elizabeth; Simeon and Anna. They came to light when the time was right. (See the beginning of Luke). After Christ had been crucified there was one who was ready to ask for the body of Jesus - Joseph of Arimathaea. Nicodemus was also involved (John 19:38-40).
If we go to the Acts we find that there was a youth, Paul's Sisterís son, who was used to bring to the notice of the chiliarch the plot that had been set on foot to kill Paul. Very likely the youth was nervous because we find that the chiliarch took him by the hand (Acts 23:16-22). The fact that it was a youth bringing the plot to his notice, may well have convinced the chiliarch that the information was certainly true.
Christ Himself was God's great reserve. He sent Him forth when the time was right (Galatians 4:4). Previously He had been hidden: "He hath concealed me under the shadow of his hand... in his quiver hath he hidden me." (Isaiah 49:2).
Further, things have often to be held back. God Himself does not bring everything out at once. Some things were left to New Testament days before they were revealed (Romans 16:25-27 and Ephesians 3). Christ also had to keep some things back because his disciples could not bear them before the Spirit of truth had come (John 16:12/13). Further Paul had to give the Corinthians milk rather than solid food because of their being yet carnal (1 Corinthians 3:1-3).
We may also have to keep things back; "A fool uttereth all his mind; but a wise [man] keepeth it back" (Proverbs 29:11).
Finally, let us remember that for the Christian there is: "an incorruptible and undefiled and unfading inheritance, reserved in [the] heavens" (1 Peter 1:4). However, for the wicked there "has been reserved the gloom of darkness for eternity" (Jude 13).