Islam Nusantara refers to the Sunni Islamic practices and preaching in the Indonesian archipelago by indigenous preachers, characterised by moderation, tolerance, and cultural adaptability. While claimed to be firmly rooted in Indonesian Islam, the concept has sparked debate between supporters and opponents. This study aims to investigate the perspectives of Indonesian Muslim intellectuals, religious leaders, and ulama on Islam Nusantara by comprehensively understanding their viewpoints and analysing their justifications. The study adopts an exploratory qualitative design to understand participants' perspectives on the phenomenon, using semi-structured interviews for data collection and thematic analysis for the analysis. The analysis revealed three major themes: i) various conceptualisations and attitudes about Islam Nusantara, ii) various normative Islamic religious ideas, and iii) non-normative attitudes. Firstly, participants' conceptualisations diverged into Islamic normative and social science perspectives. From a normative view, supportive participants understood Islam Nusantara as a contextualised version of universal Islam, a development of moderate Indonesian fiqh, or Indonesia's moderate Islamic paradigm countering extremism. From the lens of Social Sciences, Islam Nusantara was viewed as cultural Islam "Islamicate" or NU's political strategy. Participants' attitudes toward Islam Nusantara ranged from supportive to critical and neutral, supported by arguments. Supportive participants argued for Islam Nusantara's legitimacy urgency and that the disagreements were due to misunderstandings. For its legitimacy, supportive participants emphasised the contextualisation of Islam. Regarding urgency, they viewed it as crucial for countering fundamentalism threatening nationalism and harmony, refuting associations with Liberal Islam or political opportunism. Critics argued that Islam Nusantara presents a reductionist view of universal Islam, promoting religious pluralism and contradicting Islamic principles. Critical participants raised concerns over sectarianism, divisions between Islam Nusantara and other Islamic interpretations weakening Muslim unity and accommodating a "soft" Islam. A distinct group adopted a neutral stance, sharing sentiments of support and critique. This neutral attitude stemmed from a social sciences perspective, approaching Islam Nusantara as a societal phenomenon rather than normatively. Their diverse perspectives, shaped by social sciences considerations and philosophical frameworks, highlighted the debate's complexities, necessitating a comprehensive multi-perspective engagement with Islam Nusantara. Secondly, participants' attitudes were shaped by their normative religious understandings of key Islamic discourse concepts, namely the local-universal dynamic, ahl al-Sunnah wa al-jamā'ah (ASWJ) designation, religious authority's nature, Islamic law application, and uṣūl and furū' (fundamentals and branches). However, these understandings did not solely determine attitudes, as similarities and differences emerged in supportive and critical arguments. Critical participants viewed Islam Nusantara as contradicting Islam's universality, while supporters believed its contextualised nature did not negate universality, encompassing plural local expressions. Regarding ASWJ status, critical participants saw it encompassing all Sunni interpretations, disagreeing with Islam Nusantara representing it exclusively. At the same time, NU supporters affirmed NU's self-identity as ahl al-Sunnah wa al-jamā'ah, supporting Islam Nusantara's embodiment of those principles. However, arguments on textual authority, Islamic law application, and uṣūl and furū' were relatively similar between participants, regardless of their attitudes. Most adhered to classical Sunnah definitions within the uṣūl al-fiqh tradition, with few supportive participants voicing progressive interpretations. Most examined Islamic law substantively, with few supportive participants voicing formalisation, regardless of attitudes toward Islam Nusantara. Participants also shared relatively similar arguments concerning uṣūl and furū'. Thirdly, participants expressed non-normative attitudes. Critical participants viewed Islam and modernity as irreconcilable due to clashing values, language, epistemology, and power dynamics, attributing intra-Muslim conflicts to Western divisive tactics. Conversely, supporters saw Islam Nusantara reconciling Islamic tradition with modern secular values like equality and pluralism rooted in the Quran and Sunnah. Attitudes toward transnational Islamic movements varied, with some supporters holding negative views while others adopted a neutral stance, not seeing countering radicalism as Islam Nusantara's primary aim. Additionally, critical and neutral participants raised concerns over Muslim majoritarianism, arguing for preferential rights based on their majority status, from public practices to government roles and the Jakarta Charter's reinstatement. This study demonstrates the complex relationship between participants' attitudes, conceptualisations of Islam Nusantara, normative ideas, and other non-normative attitudes.