Did Joshua go with Moses up the Mountain Even though Moses Was to Go Alone?

Exodus 24:13 seems to indicate that Joshua accompanied Moses up the mountain. Wasn’t Moses instructed to go alone?

While it is true that only Moses was to “approach the Lord,” there were others who were to accompany Moses to the mountain. In light of verse 24:18, it seems clear that only Moses went up the mountain in accordance with God’s instructions (“and he stayed on the mountain forty days and forty nights”).

–Don Porter