The practice of calling upon the Lord is not only in the New Testament but also in the Old Testament. Isaiah 12:2 tells us that God Himself is our salvation and our song. Verse 3 says that He is the wells of salvation, and we have to draw water from these wells with joy. Then the following verses tell us how to draw out this water. Verse 4 says we have to praise the Lord and then call upon His name. Verse 6 says that we should cry out and shout.
How do we drink the waters of salvation? Some may say that we should quietly meditate on the Lord or study the Word. But Isaiah 12 tells us clearly that God is our salvation and our song. He is the wells of salvation. We have to draw water out of these wells not by meditating, not by being silent, but by praising the Lord, by calling upon His name, and even by crying out and shouting. According to Romans 10, if we are going to enjoy the Lord’s salvation and get into the enjoyment of all the riches of Christ, we have to exercise our mouth to call upon His name. And according to Isaiah 12, the way to draw the living water out of the wells of salvation is to praise the Lord, call upon His name, and even cry out and shout.
The Lord is the all-inclusive living Spirit. He descended from the heavens to become a man as the last Adam and ascended from the abyss to be the life-giving Spirit in resurrection (1 Cor. 15:45b). Now He is the pneuma, just like the air in your mouth. If we would open up our mouth, deep from our heart and with our spirit, to call "Lord Jesus," He would get into us. This is the way to take Christ into us, the vessels. He is the content, and we are the containers. The way for us, the containers, to take Him in as the content is to exercise our mouth to call upon His name: "O Lord Jesus! O Lord Jesus!" In this way all the bottles, the vessels, will be filled up with Christ not only as their salvation but also as all that He is in His unsearchable riches. Here Romans 10:12 says that the Lord is "rich to all who call upon Him." The way to enjoy Christ in all His unsearchable riches is to call upon Him!
(The Living and Practical Way to Enjoy Christ, Chapter 4, by Witness Lee)