Be strengthened (endunamou). Present passive imperative of endunamoō. See already 1Ti 1:12; Rom 4:20; Phi 4:13; Eph 6:10. “Keep on being empowered,” “keep in touch with the power.”
In the grace that is in Christ Jesus (en tēi chariti tēi en Christōi Iēsou). Where the power is located. Christ is the dynamo for power only when and while we keep in touch with him.