The insightful and wise do not trust that they are safe from four things: a past sin about which it is not known what the Lord Almighty will do, the remaining lifespan wherein it is not known what destruction lies, some (apparent) advantage a person is given but which could be a lure (from Allāh) in recompense for his wrongdoing, a misguidance that has been beautified – so that he thinks it is guidance – and a momentary deviation of the heart, for a person can be stripped of his religion without realizing it.
Al-Dhahabī, Siyar Aʿlām Al-Nubalāʾ 8:406.
O Yūnus! Do not try to overcome knowledge, for verily knowledge is [like vast] valleys: whichever of them you take will stop you before you traverse it. Instead, take it over the days and nights. And do not take knowledge all at once, for whoever tries to take it all at once will lose it all at once. Rather take it bit by bit over the days and nights.
Ibn ʿAbd Al-Barr, Jāmiʿ Bayān Al-ʿIlm article 652.
Whatever you fool around with, don’t fool around with your dīn (religion).
Al-Lālakāʾī, Sharḥ ʾUṣūl ʾIʿtiqād Ahl Al-Sunnah 1:163
How will you be (in the end times) when you open up your religion like the woman who opens her legs to expose her qubul not stopping anyone from coming to her?
Abū ʿAmr Al-Dānī, Al-Sunan Al-Wāridah fī Al-Fitan #241
Verily, you will find the believer (muʾmin) consistent time after time, upon one way, showing the same face; [whereas] you will find the hypocrite (munāfiq) changing colors, trying to be like everyone around him, running with every wind.
Abū Bakr Al-Daynūrī, Al-Mujālasah wa Jawāhir Al-ʿIlm #1936
You will remain upon good as long as you do not start approving of what you used to repudiate and as long as you do not repudiate what you used to know (to be right), and as long as your scholar (the knowledgeable) can speak amongst you without fear.
Ibn Baṭṭah, Al-Ibānah Al-Kubrā ḥadīth 26.