'Firmly' in the Bible

Now it will come to pass thatIn the last daysThe mountain of the house of the LordWill be [firmly] established as the highest of the mountains,And will be exalted above the hills;And all the nations will stream to it.

This is what the Lord says,“To the eunuchs who keep My SabbathsAnd choose what pleases Me,And hold firmly to My covenant,