But take diligent heed. This passage is a quotation from the Book of Deuteronomy (Joshua 6:5; Joshua 10:12; Joshua 11:13, Joshua 11:22; 30:6, 16, 20, etc) The expressions, as Keil well remarks, are "crowded together, so that obedience to God's commands may be the more deeply impressed on their hearts."
It is worthy of remark, that while beginning with the love of God, Joshua does not end there. The best proof of love is our conduct towards the person loved. If love be genuine, it is the practical principle which produces diligent service, punctual obedience, faithful attachment, the devotion of the heart and soul.
Commandment and law. The first of these words, derived from a root signifying to set up, has rather the force of what we call a positive precept, referring to single acts. The word translated law, derived from the root to cast, hence to stretch out the hand, to point out, refers rather to moral precepts.
The Greek νόμος and our law are used in the same sense. Cleave unto Him. The Hebrew is stronger, cleave into Him, as though regarding not so much isolated actions as principles of life. Our life was to be "rooted and grounded," to use an apostolic phrase, in His.
But the full significance of these words could not be understood till One had come who enabled us by faith to "eat His flesh and drink His blood," and so be united to Him as the branch to its root.