Mark 14:24
Each year in the spring, when we observe the Passover, we partake of the wine, which symbolizes the shed blood of Jesus Christ. When He instituted this new Passover symbol, He explained it by saying, "This is My blood of the new covenant, which is shed for many" (Mark 14:24). When we come under the blood of Jesus Christ upon our baptism and acceptance of Him as our Savior, we enter into that New Covenant.
Considering how important the New Covenant is, there are surprisingly few references to it within the New Testament—and even fewer explanations. What references there are—largely in the book of Hebrews—depend heavily on quotations from the Old Testament prophets. In fact, the New Testament teaching of the New Covenant relies entirely on one already having an understanding from reading the prophets. Without those prophecies, we would not understand what we actually enter into.
The New Covenant is mentioned in numerous places (Isaiah 54:9-10; 55:3; 59:20-21; 61:1-11; Jeremiah 32:36-41; 50:4-5; Ezekiel 16:60-63; 20:33-38; 34:23-26; 37:15-28), but perhaps the clearest description—and the one quoted in Hebrews 8:8-12 and 10:16-17—is found in Jeremiah 31:31-34.