In biblical theology, a "sign of covenant" refers to a physical or symbolic act or object that serves as a tangible reminder of a covenant between God and humanity. Covenants in the Bible are solemn agreements or promises made by God, often accompanied by specific signs to signify the commitment and to remind the parties involved of their obligations and promises.

1. The Rainbow:

The first explicit sign of covenant in the Bible is the rainbow, given to Noah after the flood. In Genesis 9:12-17, God establishes a covenant with Noah and all living creatures, promising never to destroy the earth with a flood again. The rainbow serves as the sign of this covenant:

"And God said, 'This is the sign of the covenant I am making between Me and you and every living creature with you, a covenant for all generations to come: I have set My rainbow in the clouds, and it will be a sign of the covenant between Me and the earth.'" (Genesis 9:12-13)

2. Circumcision:

Circumcision is the sign of the covenant between God and Abraham, as described in Genesis 17. God commands Abraham to circumcise every male in his household as a perpetual sign of the covenant:

"This is My covenant with you and your descendants after you, which you are to keep: Every male among you must be circumcised. You are to undergo circumcision, and it will be the sign of the covenant between Me and you." (Genesis 17:10-11)

3. The Sabbath:

The Sabbath is given as a sign of the covenant between God and the Israelites. In Exodus 31:16-17, the observance of the Sabbath is described as a perpetual covenant and a sign of the relationship between God and His people:

"The Israelites must keep the Sabbath, celebrating it as a permanent covenant for the generations to come. It is a sign between Me and the Israelites forever; for in six days the LORD made the heavens and the earth, but on the seventh day He rested and was refreshed." (Exodus 31:16-17)

4. The Blood of the Covenant:

In the Mosaic covenant, the blood of sacrificed animals serves as a sign of the covenant. In Exodus 24:8, Moses sprinkles the blood on the people as a sign of their agreement to the covenant:

"So Moses took the blood, sprinkled it on the people, and said, 'This is the blood of the covenant that the LORD has made with you in accordance with all these words.'" (Exodus 24:8)

5. The New Covenant:

In the New Testament, Jesus introduces the New Covenant during the Last Supper. The bread and wine serve as signs of this covenant, representing His body and blood. In Luke 22:20, Jesus states:

"In the same way, after supper He took the cup, saying, 'This cup is the new covenant in My blood, which is poured out for you.'" (Luke 22:20)

Throughout the Bible, signs of covenant serve as powerful reminders of God's promises and the responsibilities of His people. These signs are not merely symbolic but are integral to the relationship between God and humanity, marking significant theological and historical moments in the biblical narrative.