Being strong can be challenging.  God recognises this and has recorded examples and exhortations to help us.  The words pictured are the words that the Lord spoke to Joshua and the encouragement of the Holy Spirit through his servant Paul.  Both these saints experienced extremely difficult situations and both remained strong in the Lord and the power of his might (Eph 6:10).  How amazing it is that the same God who strengthened them is also our God (and Father) who will keep and protect us (Psa 121:7-8) and give us the power overcome any trial (Rom 8:31) according to his will and purposes.