Matthew 18:10 states, “See that you do not look down on one of these little ones. For I tell you that their angels in heaven always see the face of my Father in heaven.” In the context, “these little ones” could either apply to those who believe in Him (v. 6) or it could refer to the little children (vs. 3-5). This is the key passage regarding guardian angels. There is no doubt that good angels help protect (Daniel 6:20-23; 2 Kings 6:13-17), reveal information (Acts 7:52-53; Luke 1:11-20), guide (Matthew 1:20-21; Acts 8:26), provide for (Genesis 21:17-20; 1 Kings 19:5-7), and minister to believers in general (Hebrews 1:14).
The question is whether each person—or each believer—has an angel assigned to him/her. In the Old Testament, the nation of Israel had the archangel (Michael) assigned to it (Daniel 10:21; 12:1), but Scripture nowhere states that an angel is “assigned” to an individual (angels were sometimes sent to individuals, but there is no mention of permanent assignment). The Jews fully developed the belief in guardian angels during the time between the Old and New Testament periods. Some early church fathers believed that each person had not only a good angel assigned to him/her, but a demon as well. The belief in guardian angels has been around for a long time, but there is no explicit scriptural basis for it.
To return to Matthew 18:10, the word “their” is a collective pronoun in the Greek and refers to the fact that believers are served by angels in general. These angels are pictured as “always” watching the face of God so as to hear His command to them to help a believer when it is needed. The angels in this passage do not seem to be guarding a person so much as being attentive to the Father in heaven. The active duty or oversight seems, then, to come more from God than from the angels, which makes perfect sense because God alone is omniscient. He sees every believer at every moment, and He alone knows when one of us needs the intervention of an angel. Because they are continually seeing His face, the angels are at His disposal to help one of His “little ones.”
It cannot be emphatically answered from Scripture whether or not each believer has a guardian angel assigned to him/her. But, as stated earlier, God does use angels in ministering to us. It is scriptural to say that He uses them as He uses us; that is, He in no way needs us or them to accomplish His purposes, but chooses to use us and them nevertheless (Hebrews 1:7). In the end, whether or not we have an angel assigned to protect us, we have an even greater assurance from God: if we are His children through faith in Christ, He works all things together for good (Romans 8:28-30), and Jesus Christ will never leave us or forsake us (Hebrews 13:5-6). If we have an omniscient, omnipotent, all-loving God with us, does it really matter whether or not there is a finite guardian angel protecting us?