That thou stir up the grace of God. In the Greek is a metaphor for fire that is blown up again.
Which is in thee by the imposition of my hands, when thou wast ordained bishop. (Witham)
The grace, which St. Paul here exhorts Timothy to stir up in him, was the grace he had received by imposition of hands, either in his confirmation, or at receiving the sacrament of orders, being a bishop. This verse seems to show that the imposition of hands is used in these two sacraments, as the essential matter of the sacraments, being the instrumental cause of the grace therein conferred. (Denis the Carthusian)
You see how greatly dispirited and dejected he considers him to be. He almost says, Think not that I despise you, but be assured that I do not condemn you, nor have I forgotten you. Consider, at any rate, your mother and your grandmother. It is because I know that you have unfeigned faith that I put you in remembrance. For it requires much zeal to stir up the gift of God. As fire requires fuel, so grace requires our alacrity, that it may be ever fervent. I put you in remembrance that thou stir up the gift of God, that is in you by the putting on of my hands, that is, the grace of the Spirit, which you have received, for presiding over the Church, for the working of miracles, and for every service. For this grace it is in our power to kindle or to extinguish; wherefore he elsewhere says, Quench not the Spirit. 1 Thessalonians 5:19 For by sloth and carelessness it is quenched, and by watchfulness and diligence it is kept alive. For it is in you indeed, but do thou render it more vehement...
For it requires much zeal to stir up the gift of God. As fire requires fuel, so grace requires our alacrity, that it may be ever fervent…. For it is in our power to kindle or extinguish this grace…. For by sloth and carelessness it is quenched, and by watchfulness and diligence it is kept alive. For it is in you indeed, but you must render it more vehement, that is, fill it with confidence, with joy and delight. Stand manfully.
