The fact that this verb is in the passive expresses the idea that the one who is doing the storing is God, not the believer. This hope is not dependent on man, but on God. Moo expresses the same idea: the fact that this hope is stored up for them has the implication that it does not depend on them; someone has set it aside for them. Christian hope rests in God, the one who has secured this hope in Christ.1
5 because of the hope laid up for you in heaven. Of this you have heard before in the word of the truth, the gospel,