Belief in the Messengers implies four things:

1 – Firm belief that Allaah sent to every nation a Messenger from amongst them, to call them to worship Allaah alone and to disbelieve in everything that is worshipped instead of Him, and that they (the Messengers) were all truthful, speakers of the truth, righteous, wise, pious and honest, and that they conveyed everything with which Allaah sent them, concealing and altering nothing. They did not add even one letter from themselves or omit anything. 

“Are the Messengers charged with anything but to convey clearly the Message?”

[al-Nahl 16:35 – interpretation of the meaning]

Their message was the same, from the first to the last of them, based on the principle of Tawheed, i.e., that all kinds of worship should be devoted to Allaah alone, beliefs, words and deeds alike, and that everything that is worshipped instead of Allaah is to be rejected. The evidence for that is the verses (interpretation of the meanings): 

“And We did not send any Messenger before you (O Muhammad) but We revealed to him (saying): Laa ilaaha illa Ana [none has the right to be worshipped but I (Allaah)], so worship Me (Alone and none else)”[al-Anbiya’ 21:25]

“And ask (O Muhammad) those of Our Messengers whom We sent before you: ‘Did We ever appoint aalihah (gods) to be worshipped besides the Most Gracious (Allaah)?’”[al-Zukhruf 43:45]

And there are very many similar verses. 

With regard to the obligatory duties by which Allaah is to be worshipped and the minor details of legislation, prayers and fasts may have been enjoined on some nations but not on others, and some things may have been forbidden to some and permitted to others, as a test from Allaah. 

“Who has created death and life that He may test you which of you is best in deed”
[al-Mulk 67:2 – interpretation of the meaning]

The evidence for that is in the verse (interpretation of the meaning): 

“To each among you, We have prescribed a law and a clear way”

[al-Maa’idah 5:48]

Ibn ‘Abbaas (may Allaah be pleased with him) said “(This means) a way and a path.” Mujaahid, ‘Ikrimah and many of the mufassireen (commentators) said likewise. 

In Saheeh al-Bukhaari (3443) and Saheeh Muslim (2365) it is narrated that Abu Hurayrah said: “The Messenger of Allaah (peace and blessings of Allaah be upon him) said: ‘The Prophets are like brothers from one father, their mothers are different but their religion is one’” – i.e., the Prophets are united on the principle of Tawheed, the message with which Allaah sent every Messenger whom He sent, and which He included in every Book that He revealed, but their laws differed as to command and prohibitions, what was permitted and what was forbidden.  

Whoever disbelieves in the message of one of them has disbelieved in all of them, as Allaah says (interpretation of the meaning): 

“The people of Nooh (Noah) belied the Messengers”

[al-Shu’ara’ 26:105]

Allaah described them as disbelieving in all of the Messengers even though there was no Messenger other than Nooh at the time when they disbelieved. 

2 – Belief in those Messengers whose names we know, such as Muhammad, Ibraaheem, Moosa, ‘Eesa and Nooh (may blessings and peace be upon them all). With regard to those who have been mentioned in general terms but whose names we do not know, we are obliged to believe in them in general terms, as Allaah says (interpretation of the meaning): 

“The Messenger (Muhammad) believes in what has been sent down to him from his Lord, and (so do) the believers. Each one believes in Allaah, His Angels, His Books, and His Messengers. (They say,) ‘We make no distinction between one another of His Messengers’”

[al-Baqarah 2:285]

“And, indeed We have sent Messengers before you (O Muhammad), of some of them We have related to you their story. And of some We have not related to you their story”

[Ghaafir 40:78]

And we believe that the Final Messenger was our Prophet Muhammad (peace and blessings of Allaah be upon him), and there is no Prophet after him, as Allaah says (interpretation of the meaning): 

“Muhammad is not the father of any of your men, but he is the Messenger of Allaah and the last (end) of the Prophets. And Allaah is Ever All-Aware of everything”[al-Ahzaab 33:40]

In al-Bukhaari (4416) and Muslim (2404) it is narrated from Sa’d ibn Abi Waqqaas (may Allaah be pleased with him) that the Messenger of Allaah (peace and blessings of Allaah be upon him) went out to Tabook, and appointed ‘Ali as his deputy (in Madeenah). ‘Ali said, “Are you leaving me in charge of the children and women?” The Prophet (peace and blessings of Allaah be upon him) said, “Does it not please you that you are to me like Haroon to Moosa? But there will be no Prophet after me.” 

Allaah favoured him and singled him out for great blessings which were given to no other Prophet. For example: 

1-Allaah sent him to all of the two races of mankind and jinn, whereas each Prophet who came before him was sent only to his own people.

2-Allaah supported him against his enemies by striking fear into the hearts of all his enemies within the radius of one month’s travel.

3-The earth was made a place of prayer and a means of purification for him.

4-War booty was made permissible for him, where it had not been permitted to any Prophet before him.

5-He will be granted the position of greater intercession.

And there are many other special favours that have been bestowed upon him (peace and blessings of Allaah be upon him). 

3 – Believing in the sound reports that have been narrated from the Messengers. 

4 – Following the laws of the Messenger who has been sent to us, namely the Final Prophet Muhammad (peace and blessings of Allaah be upon him), who was sent to all of mankind. Allaah says (interpretation of the meaning): 

“But no, by your Lord, they can have no Faith, until they make you (O Muhammad) judge in all disputes between them, and find in themselves no resistance against your decisions, and accept (them) with full submission”[al-Nisa’ 4:65]

We should note that belief in the Messengers bears great fruit, such as: 

1-Knowledge of the mercy of Allaah towards His slaves and His care for them, as He sent the Messengers to guide them to the path of Allaah and to show them how to worship Allaah, for human reason alone cannot come to know that.

2-Giving thanks to Him for this great blessing.

3-Loving and venerating the Messengers (blessing and peace be upon them), and praising them in the manner that befits them, because they are the Messengers of Allaah, and because they worshipped Him and conveyed His Message, and were sincere towards His slaves.

And Allaah knows best. 

See A’laam al-Sunnah al-Manshoorah, 97-102; Sharh Usool al-Thalaathah by Shaykh Ibn ‘Uthaymeen, 95, 96.