It is obvious that Old Testament scripture portrays angels as God's ministers who, at times, were given special commissions to intercede on the behalf of men and to perform other missions. Psalm 91:9-11 says:
If you make the Lord your refuge,
if you make the Most High your shelter,
no evil will conquer you;
no plague will come near your home.
For he will order his angels
to protect you wherever you go.
Although there is no specific Old Testament teaching, regarding the exact relationship between angels and mankind, scriptures like Psalm 91 do indicate a common belief that angels, under instruction from God, do intercede in the lives of humans.
The concept of ministering angels is more fully developed in the New Testament, which portrays angels as intermediaries between God and mankind. One of the most quoted ministering angel scriptures is Matthew 18:10: "Beware that you don't look down on any of these little ones. For I tell you that in heaven their angels are always in the presence of my heavenly Father." Most people relate this to guardian angels for children but the context of the scripture makes it just as probably that the scripture is talking about those who are saved. This is especially true if you insert verse 11, which is not present in all old manuscripts. There is also scripture where the author of Hebrews is explaining that Jesus is greater than angels and he makes the statement: "Therefore, angels are only servants—spirits sent to care for people who will inherit salvation." (Hebrews 1:14) Based on these and other scripture we can safely say that the New Testament gives us assurance that angels, under instruction from God, do intercede in the lives of humans.
Angels interceding on the behalf of man is not exactly the same thing as our having a personal guardian angel. The bulk of guardian angel belief came from early Catholic writers such as St. Thomas, St. Basil, St. Chrysostom, the medieval liturgical writer William Durandus, and the Franciscan John Duns Scotus. These men went beyond scripture and in so doing built the foundation on which our current guardian angel teaching rests. This is not to say that they are in error. It is simply to point out that early Catholic leaders took scripture and through their inductions and deductions constructed the concept of our having personal guardian angels.
Can we say with absolute certainty that every single born again Christian has a guardian angel? No! Can we say with absolute certainty that every single born again Christian does not have a guardian angel? No! We can say with absolute certainty, however, that angels do have a mission to care for people who are born again. It really does not matter if we have one individual angel assigned to us or if all the angels are responsible for carrying out God's instructions to take care of us. The most important thing for us to realize is that God loves us and thus we are not alone.