Christians from NT times have always referred to both Jesus Christ and the Father as "Lord". The Bible used by first-century Christians, the Septuagint, rendered Hebrew Yahweh with kurios "lord," so that all references to Yahweh in the OT became references to the "Lord". Many modern Bible translations continue this, rendering the tetragrammaton with "LORD" in capitals. The King James Version does this as well and uses "Jehovah" only in four instances. Because kurios was used to refer to God in the OT, and because Jesus was kurios, OT passages that refer to God were applied by Paul and other NT writers to Jesus. A good example of this is Romans 10:
"If you confess with your mouth, ?Jesus is Lord,? and believe in your heart that God raised him from the dead, you will be saved. For it is with your heart that you believe and are justified, and it is with your mouth that you confess and are saved. As the Scripture says, ?Anyone who trusts in him will never be put to shame." For there is no difference between Jew and Gentile ? the same Lord is Lord of all and richly blesses all who call on him, for, ?Everyone who calls on the name of the Lord will be saved." (Romans 10:9-13).
This verse plainly applies the scripture in Joel 2:23 to refer to Jesus Christ, even though the "Lord" mentioned in this OT text was Yahweh. Paul says that "If you confess 'Jesus is Lord' you will be saved," and this alludes clearly to Joel 2:23 (LXX) and makes the "Lord" of this OT passage refer to Jesus. The NWT, by inserting the divine name into the NT (arbitrarily without any manuscript evidence), destroys the very simple and clear thought here that Jesus is the "Lord" of the OT:
"For if you publicly declare that word in your own mouth, that Jesus is Lord, and exercise faith in your heart that God raised him up from the dead, you will be saved. For with the heart one exercises faith for righteousness, but with the mouth one makes public declaration for salvation. For the Scripture says: "None that rests his faith on him will be disappointed." For there is no distinction between Jew and Greek, for there is the same Lord over all, who is rich to all those calling upon him. For "everyone who calls on the name of Jehovah will be saved" (NWT).
The exact same application of OT "Jehovah" to Jesus occurs in Acts 2:16-21. There are many, many other cases of scriptures referring to God in the OT that are applied to Jesus -- as both make reference to the "Lord". Compare, for instance, Matthew 3:1-3 with Isaiah 40:3 and ask who John the Baptist was preparing a way for, or Matthew 22:41-46 with Psalm 110:1 which it quotes, or 1 Peter 2:6-8 citing Isaiah 8:13-15 (in which the subject is "Jehovah of armies"), Revelation 6:15-17 citing Isaiah 2:19 (applying what referred to "Jehovah" to the "Lamb"), or Philippians 2:10-11 citing Isaiah 45:23, and so forth.