I am not aware of any direct instruction of avoiding the blessing of nonbelievers of God within His Word specifically. However, a mature and growing Christian will have a different take on this subject, as compared to a young Christian, and both will think much differently on the matter when compared to a person of the world. It’s not so much a matter of written rules, as it is the total relationship a person has with the LORD GOD. If you are a person of the world, and offer blessings to nonbelievers as a way to show that you are a caring person, then it is not wrong. That person would not be invoking the LORD GOD’s response, and the words might bring comfort.
If you are a young Christian, and just beginning to walk the walk of Jesus Christ, and trying hard to love in accordance with the standards that he has set, and are moved by him to express those first steps of love, then the offering of your heart felt blessing to a lost soul might not be wrong. Sometimes the beginning joyful walk with Christ leads many unable to distinguish the different types and responses to love that a mature Christian develops, and responding with a sincere blessing to a nonbeliever might be a result. In fact, Jesus instructs all of his followers:
Matthew 5:43-48
“43 ¶ Ye have heard that it hath been said, Thou shalt love thy neighbour, and hate thine enemy. 44 But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you; 45 That ye may be the children of your Father which is in heaven: for he maketh his sun to rise on the evil and on the good, and sendeth rain on the just and on the unjust. 46 For if ye love them which love you, what reward have ye? do not even the publicans the same? 47 And if ye salute your brethren only, what do ye more [than others]? do not even the publicans so? 48 Be ye therefore perfect, even as your Father which is in heaven is perfect.” (Mt 5:43-48 AV)
Bless your enemies are his instructions, and we must follow Christ’s instructions exactly. But what about the haters of God? What about the blasphemers and the idolaters and the deniers that have no thought or action toward the LORD God except for disdain and hatred? Does Jesus want Christians asking for His blessings to be bestowed upon the heathen and the worldly wicked.
If you are a mature Christian, with a solid foundation of Jesus Christ, and a dedicated relationship with the LORD God, then you are confident and reassured throughout His Word that he hears your prayers, and responds with His Will to your requests. You know that your word matters to God, and you are assured that your message will be delivered to His ears.
If a faithful and strong Christian were to bless a nonbeliever, he would be wrong, in most cases. By requesting the blessing of God to come upon a person, that Christian is asking God for that specific thing. Spiritually speaking, there are very few cases, if any, that He would be pleased that you were blessing someone that hated Him.
Praying for the nonbelievers to receive sight and hearing so they might turn to Him is one thing, but to ask for his blessings to be directed to the unbeliever seems insulative and blatantly using God’s name and nature to benefit only yourself with the world watching on. To presume that blessings should go to any non-believer or hater of God immediately places a Christian in a precarious place. If our Father were to listen and respond to the misguided Christian’s request for a blessing to the ungodly, then He would certainly suspect that Christian of not loving or walking in truth toward Him.
Nobody that loves somebody deeply wants to betray. We know the LORD God’s way of rewarding obedience and punishing disobedience. Those not belonging to Him, those not fearing and worshipping Him, are not going to be receiving the best parts of Him, and for a Christian to presume otherwise is wrong.
So it’s a fine, fine, fine grey line, and a very narrow path we sometimes have to walk. If we are being attacked by our enemies, then we are to pray for them, and offer blessings instead of curses. However, strong Christian workers must avoid placing themselves in situations where the haters of God can trick them for a blessing from practiced speech, expected responses or patterns of Christian behavior.
What we say as Christians matters to our God and our Savior. The words that come from our mouths are noted and acted upon according to Their Will. If the wicked sneeze, there is nothing in the Bible that tells us to say “God bless you”, and to offer that blessing makes you a fool. We are not expected to pray for blessings on the heathen, nor ask reward for the haters of God. On the other hand, if the wicked curse you, the Bible is clear that you are to bless them, and not to do so makes you disobedient to Christ.
It’s all about love in the end. Love your enemies when they attack you and fight the way of Christ, but for God’s sake, don’t forget to love Him also! To love the LORD God with all your heart and all your soul and all your might requires you to reject the wicked worldly who hate Him and His Way! If you are in the presence of the wicked worldly and they sneeze, hand them a tissue and keep your mouth shut.