God is the one who strengthens us. However, Paul identifies what God strengthens and for what purpose. 

”May he strengthen your hearts so that you will be blameless and holy in the presence of our God and Father when our Lord Jesus comes with all his holy ones.“ 1 Thessalonians‬ ‭3‬:‭13‬ ‭NIV‬‬

When Jesus returns, He is coming back for His bride. We are to prepare ourselves. 

”Let us rejoice and be glad and give him glory! For the wedding of the Lamb has come, and his bride has made herself ready. Fine linen, bright and clean, was given her to wear.” (Fine linen stands for the righteous acts of God’s holy people.)“. Revelation‬ ‭19‬:‭7‬-‭8‬ ‭NIV‬‬