Those are excellent questions! When I think of this ‘saying’ I think of the command that God gave to the priests who served Him in the tabernacle in the wilderness as they made their way to the Promised Land. Let’s read Numbers 6:22-27, “And the LORD spoke to Moses, saying: ‘Speak to Aaron and his sons, saying, This is the way YOU SHALL BLESS THE CHILDREN OF ISRAEL. Say to them: ‘The LORD BLESS YOU and keep you; the LORD make His face shine upon you, and be gracious to you; the LORD lift up His countenance upon you, and give you peace.’ So they shall put My name on the children of Israel, and I WILL BLESS THEM” (NKJV). It was always in the heart of God to bless His people and thus He commanded the priests to utter this “benediction” to His people. If we read the first six chapters of Numbers we would see that God’s blessing them would be conditioned upon their obedience to Him. If the people were walking according to His commandments, God would continue to BLESS THEM; that is, He would continue to SHOW FAVOR TO THEM by meeting their every need. It was the privilege of Aaron and his sons to “pronounce the favor of God upon them” in these beautiful words.

This well-known BENEDICTION is still cited throughout Christendom today as Christians meet together for worship and ministry. Is it right for them to do so? Or, as your question stated, is it right for us to say ‘God bless you’ to an individual at any time? Let me say emphatically “God desires to bless us” and this is especially true of every born again believer in Christ. The fact is, “He has already blessed us richly,” as we see in Ephesians 1:3, “Blessed be the God and Father of our Lord Jesus Christ, WHO HAS BLESSED US with every spiritual blessing in the heavenly places in Christ.” Through the death and resurrection of Christ we have been blessed with redemption and the forgiveness of sins (see Ephesians 1:7), eternal life (John 3:16 and 5;24), justification (Romans 3:21-26), a perfect standing before God in His Son (Ephesians 1:6 and 2nd Corinthians 5:17), a home in heaven and the hope of Christ’s return to take us to glory (1st Peter 1:3-4 and John 14:2-3), and many other “spiritual blessings.” So, in that sense, we do not need to say to a fellow believer, “God bless you,” for He has already done so. Yet sadly many Christians have not entered, by faith, into the blessings God has given to them in Christ. In those cases we would be more than justified in saying to them, “The Lord bless you,” for in uttering these words we would, in essence, be hoping that they would indeed come to KNOW AND ENJOY THE HEAVENLY BLESSINGS God has blessed them with.

I believe those words also imply that we would wish ALL OF GOD’S BLESSINGS upon them, not just “spiritual blessings,” but “temporal blessings” too. In other words, God’s blessings would include God meeting every need we have. The Lord Jesus spoke about these blessings (food, clothing, and shelter) in Matthew 6:25-34 (which I would encourage you to read) and He ended by saying, “For all THESE THINGS the Gentiles seek.  For your heavenly Father knows that you need all THESE THINGS. But seek first the kingdom of God and His righteousness, and all THESE THINGS shall be added to you.” Just as the children of Israel’s temporal blessings were conditioned upon obedience to God’s Word, our temporal blessings would be conditioned upon putting God’s interests first in our lives. In this context, when we say to a Christian, “God bless you,” we actually desire that they would seek God’s interests above all others and then experience God’s blessings as a result.

In closing, you also asked about saying ‘God bless you’ to non-Christians and that deserves an answer. The Lord Jesus addressed this subject in the same message (which we refer to as “The Sermon on the Mount” in Matthew chapters 5-7). In Matthew 5:44 He said, “Love your enemies, BLESS THOSE WHO CURSE YOU, do good to those who hate you, and pray for those who spitefully use you and persecute you.” He is not exactly instructing us to say “God bless you” to unbelievers who persecute us, but He is teaching us to DESIRE TO SEE THEM BLESSED and to BE A MEANS OF BLESSING TO THEM. If we faithfully, by God’s grace, return love for their hatred and desire them to be blessed while they curse us, we may very well be used of God in the conversion of their souls, which, in turn, will lead to them having all the blessings (spiritual and temporal) from God that we have already considered. I have heard believers say words like these to their persecutors, “I know you hate me and my Lord, but God loves you anyway and wants to bless you with His salvation.” In these words one is really saying to them, “God bless you.”  (396.5)  (DO)