Ibn Kathir in his tafsir (see here in Arabic or the source of my quote: qtafsir) says -sometimes quoting imam at-Tabari-, about the last part of this verse:
(So, We gave power to those who believed against their enemies,)
that is, "We gave them victory over the group of Christians which defied them,"
(and they became the victorious (uppermost).)
"over the disbelieving group, when We sent Muhammad." Imam Abu Ja'far bin Jarir At-Tabari reported that Ibn 'Abbas said, "When Allah decided to raise 'Isa to heaven, 'Isa went to his companions while drops of water were dripping from his head. At that time, there were twelve men at the house.
'Isa said to them, "Some of you will disbelieve in me twelve times after having believed in me." He then asked, "Who among you volunteers that he be made to resemble me and be killed instead of me; he will be with me in my place (in Paradise)." One of the youngest men present volunteered, but 'Isa commanded him to sit down.
'Isa repeated his statement and the young man again stood up and volunteered, and 'Isa again told him to sit down. 'Isa repeated the same statement and the young man volunteered. This time, 'Isa said, "Then it will be you." The appearance of 'Isa was cast upon that young man, while 'Isa, peace be on him, was raised to heaven through an opening in the roof of the house.
The Jews came looking for 'Isa and arrested the one that appeared as him, killing him by crucifixion. Some of them disbelieved in 'Isa twelve times, after they had believed in him. They divided into three groups.
One group, Al-Ya'qubiyyah (the Jacobites), said, "Allah remained with us as much as He willed and then ascended to heaven."
Another group, An-Nasturiyyah (the Nestorians), said, "Allah's son remained with us as much as Allah willed and He then rasied him up to heaven."
A third group said, "Allah's servant and Messenger remained with us as much as Allah willed and then Allah raised him up to Him."
The last group was the Muslim group. The two disbelieving groups collaborated against the Muslim group and annihilated it. Islam remained unjustly concealed until Allah sent Muhammad.
(Then a group of the Children of Israel believed and a group disbelieved.) This Ayah refers to the group among the Children of Israel that disbelieved and the group that believed, during the time of 'Isa,
(So, We gave power to those who believed against their enemies, and they became the victorious (uppermost).)
through the victory that Muhammad gained over the religion of the disbelievers, which brought the dominance of their religion.'' This is the wording in his book for the Tafsir of this honorable Ayah. Similarly, An-Nasa'i collected this statement of Ibn 'Abbas in his Sunan. Therefore, the Ummah of Muhammad will always be prevalent on the truth until Allah's command (the Final Hour) commences, while they are on this path. The last group of them will fight against Ad-Dajjal along with 'Isa, peace be on him, according to Hadiths in the authentic collections. This is the end of the Tafsir of Surat As-Saff. All praise and thanks are due to Allah.
This means one of three "groups pr sects" from among the Christians or the former followers of 'Isa () was still believing that he is only a Prophet or Messenger and this group gained strength -as their belief will be shown as true- by the beginning of the mission of Muhammad (), as he will confirm their beliefs.
Note that the sunan of an-Nasa'i which ibn Kathir is refereing to are as-Sunan al-Kubra (not the sunan -as-Sughra- which we know and may find in sunnah.com and are among the so called 6 books) you may find this hadith in the book of Tafsir (54) hadith #11527 (see here in Volume 10, Chapter 54 page 298-299).
Imam al-Qurtubi in his tafsir said (My own translation take it carefully):
قال ابن عباس : أيد الله الذين آمنوا في زمن عيسى بإظهار محمد على دين الكفار
that ibn 'Abas said that those whom believed at the time of 'Isa have been risen by Allah by the victory of Muhammad () over the kufaar (disbelievers.
وقال مجاهد : أيدوا في زمانهم على من كفر بعيسى . وقيل أيدنا الآن المسلمين على الفرقتين الضالتين ، من قال : كان الله فارتفع ، ومن قال : كان ابن الله فرفعه الله إليه ; لأن عيسى ابن مريم لم يقاتل أحدا ولم يكن في دين أصحابه بعده قتال
He also quoted a few of Mujahid's interpretations which are somewhat simiar to what I've quoted above from ibn Kathir's tafsir: They have been risen or supported at their time over those who disbelieved 'Isa, and it was said that WE (Allah) have risen the believers (Muslims) over the two wrong groups who went astray: those who said he was God and rose and those who said he was the son of God and was risen by God, as 'isa didn't fight anybody nor was it part of the religion of those who followed him to fight afterwards.
وقال زيد بن علي وقتادة : " فأصبحوا ظاهرين " غالبين بالحجة والبرهان ; لأنهم قالوا فيما روي : ألستم تعلمون أن عيسى كان ينام والله لا ينام ، وأن عيسى كان يأكل والله تعالى لا يأكل !
Zaid ibn 'Ali and Qatada said about (and they became the victorious (uppermost).) they became victorious by proof and evidence, as they said as it was narrated: "Don't you know that 'Isa used to sleep while Allah doesn't sleep, And 'Isa used to eat, while Allah doesn't eat!"
So far this is also quoted by al-Fakhr ar-Razi in his tafsir.
Al-Qurtobi also quoted ibn Isahaaq who said that the verse was addressing the messenger of 'Isa to other nations, he (or his source ibn Ishaaq) quoted names of messengers to several nations and places like Babylon, Carthage the Berber, al-Hijaz, Alexandria etc.
Ibn 'Ashur in his great tafsir at-tahriri wa-tanwir added a few meanings:
Explaining that this verse shows that the few believers whom truly followed 'Isa have been "victorious" over the disbelievers and other Jew whom rejected him, by a great follow ship.
And the meaning behind this verse for the Muslims is a vow or a promise of Allah that if they are truly are helpers of Allah أنصار الله as it was ordered to them Allah will help them and give them the victory over the disbelievers.
Note that this promise is related to the first verses of this surah (61:2-3 ...) which address those who say something while they don't truly do so, which is regarded as the most hateful thing by Allah (as it is nifaaq):
O you who have believed, why do you say what you do not do? (2)
Great is hatred in the sight of Allah that you say what you do not do. (3)
which are a good description of attributes of hypocrites (monafiq):
- The Prophet (ﷺ) said, "Whoever has the following four (characteristics) will be a pure hypocrite and whoever has one of the following four characteristics will have one characteristic of hypocrisy unless and until he gives it up.
Whenever he is entrusted, he betrays.
Whenever he speaks, he tells a lie.
Whenever he makes a covenant, he proves treacherous.
Whenever he quarrels, he behaves in a very imprudent, evil and insulting manner."
Allah's Messenger (ﷺ) said, "The signs of a hypocrite are three: Whenever he speaks, he tells a lie; and whenever he promises, he breaks his promise; and whenever he is entrusted, he betrays (proves to be dishonest)".
Three are the signs of a hypocrite, even if he observed fast and prayed and asserted that he was a Muslim.
so only true believers can count on Allah's promise, as Allah truly does what he promised them!
Imam az-Zamakhshari in his tafsir al-Kashaf quoted a hadith (I've never heard before and couldn't find neither source nor full chain for) I also found quoted the same way in tafsir al-Baydawi:
عن رسول الله صلى الله عليه وسلم: " من قرأ سورة الصف كان عيسى مصليا عليه مستغفرا له ما دام في الدنيا وهو يوم القيامة رفيقه ".
The hadith says (my own words and translation):
Him who recites the surah as-Saff (61), 'Isa will send blessings to him and asking Allah for his forgivness (for the reciter) in this life and he will be his companion in the day of Judgment.