Jesus promised to send the Holy Spirit to lead the church to “all truth”.
Indeed He did!
I have yet many things to say unto you, but ye cannot bear them now. Howbeit when he, the Spirit of truth, is come, he will guide you into all truth: for he shall not speak of himself; but whatsoever he shall hear, that shall he speak: and he will shew you things to come. He shall glorify me: for he shall receive of mine, and shall shew it unto you. All things that the Father hath are mine: therefore said I, that he shall take of mine, and shall shew it unto you.
John 16:12-15 KJV
This was part of what Christ said after the Last Supper, so it is addressed to the Apostles. How do you know that it applies to *all* followers of Christ, even future ones?