God, who knows the heart, acknowledged them by giving them the Holy Spirit, just as He did to us, and made no distinction between us and them, purifying their hearts by faith.
Acts 15:8-9

The Spirit Himself bears witness with our spirit, that we are children of God, and if children, then heirs -- heirs of God and joint heirs with Christ.
Romans 8:16-17

Christians have assurance of their eternal salvation because the Holy Spirit teaches them this in their hearts. If you are not certain of this assurance, then the question of your place in heaven may yet be unsettled.