Christ is our only Mediator (1 Tim. 2:5). There is no mediation apart from Christ. But as members of the mystical body of Christ, we can also mediate for others along with Christ, the head. In John's gospel, we read, "I am the vine, you are the branches. He who abides in me, and I in him, bears much fruit" (15:5). Since the saints abide in Jesus in a special way, they are capable of mediating for us.
As God the father has sent Jesus, Jesus sends his disciples to reach out the whole world (Jn. 20:21). In his epistles, St. Paul often requests the intercession of the believers, "Pray always; with all prayer and supplication for all the saints, and for me, that utterance may be given to me, that I may open my mouth boldly to make known the mystery ofthe gospel (Eph. 6:18-20). If the intercession of the living is so powerful (ct. 1 Tim 2:1-4), the intercession of the dead who are united with Christ, is more rewarding.
It was St. Paul who wrote that there is only one Mediator, that is Christ (1 Tim 2:5). The same Paul requests our intercessory prayer for all and especially for our authorities (1 Tim 2: 1-4). He also requests peoples' intercession for himself (Eph. 6:18-20). Putting these together, we infer that our mediation or mediation of Saints are not against Christ's mediation. When we mediate we are mediating in, through and with Christ.
In the Bible there are some hints about the continuous prayer of the saints after their death. In Jer 15: 1 , the Lord says to the prophet that he would not accept the mediation of even Moses and Samuel in favour of his people. And we know Moses and Samuel died many centuries before Jeremiah. In Rev. 8:3-4, there is mention of an angel holding a golden censer whose incense he should offer with the prayers of all saints upon the golden altar which was before the throne". And the smoke of the incense, with the prayers of saints ascended before God from the angel's hand" (VA). From these references, it is sufficiently clear that the prayers of the deceased saints are continously ascending to God, and they are capable of mediating for us.
Jesus was never averse to intercessory prayers. Taking heed to the intercession of the centurian Jesus healed his servant (Mt. 8:5-13). According to the request of a Canaanite woman, Jesus healed her daughter (Mt. 15:21-28). Seeing the faith of those who brought the paralytic, Jesus healed him (Mk. 2:112), The constant prayer offered to God by the church for Peter (Acts 12:5), miraculously saved him from the prison (W 6-11). Jesus will certainly heed to our intercessory prayer and bless all those for whom we pray, Jesus has assured his presence when two or more gather in his name (Mt. 18:20). Whether this coming together takes place on earth or in heaven, Jesus' lively presence is experienced there, and he receives their prayer in his name and offer them to God the Father.